Loonook, let's roll the numbers again.
As you observed, one casting is about 128 acres. The typical family farm would be 40 acres, so one casting covers three farms.
Spell costs 150 to get cast, so 50 gp per farm. That's a gold piece a week, more or less, per farm.
That's about a ditch-digger's wage.
Increasing the yield, and potentially the profits, by a third for the cost of an unskilled hired hand isn't that far out of line.
More importantly, the fact that said farm can now devote a larger portion of the land to a cash crop, while still feeding the family, throws the deal more on the positive side.
It isn't going to be for everyone, of course, but it's going to be a factor in food production, over all.
Spells like Weather Control would, of course, be out of the question for the most part, unless needed to save a crop from drought, and since the radius on that one is in miles, the cost would likely be born by the community in general.
As a note: Draft animals don't eat grain normally. "Corn Fed" beef is a modern perversion, and is actually bad for the cattle. We do it to fatten them up quickly. They normally graze, eating grass.
Your assumption on land is incorrect:
A decent summation of Medieval Farming from a Hundred Years War game said:
In England, the idea farm size for a family was a "yardland" (24-30 acres) in size. Only about a quarter of the English farm families had this much land (or a bit more) before the Bubonic Plague , most had ten or fewer. Those farmers possessing a yardland were able to work their land efficiently enough to feed themselves and prodice a surplus for sale.
Let us split the difference and call it 20 acres. They go on to discuss the use of a supplemental vegetable garden at an acre to grow non-staple crops, land needed for the house, pigs, horses, chickens, and cattle/sheep for milk, leather, wool (with sheep) and others. Then you are also going to need homespun materials such as hemp or flax. Just for the sake of it give the farmers enough pigs to keep them in meat, and smoke hams during the fattening, and sheep or cow to turn to tallow and suet as needed along with meat, milk, leather, and wool.
Figuring based on basic info online (google
"how many acres per [blank]") and you'll find that cows need around 5 acres minimum per head, pigs around an acre per 10 (paddocking needs to be performed for both to prevent overgrazing), and sheep I would place somewhere between. For your overall animal needs we would put it at an extremely low 5 acres... Cows will overgraze your pastures, but of course you can always send your sheep to herd and summer, and pigs and cows may cycle pasture in paddocks of others who need them for their fallow field maintenance. Of course you're going to have to milk your cows at least 1/day in the morning to maintain milk production, and your piggies are going to be doing some damage around that fallow field... But each will need at least a bit of land for their own use. Your horses will also need graze, which would be around a minimum of 2-3 acres per horse. You are still allowing everyone to free-graze right?
Most medieval farms that gain better yields practiced three-crop growth cycles, leaving a third to fallow, third to grow crop A, third to grow crop B. This basic form of crop rotation cuts your production overall of course, but allows for the two crops grown to yield more over that period.
On a 20 acre farm we sacrifice around a half acre for our little home, outbuildings, smokehouse, courtyard/henhouse for your chickens, stabling for the horses, a poke for your pigs, and stalls for your cows and wintering sheep. Some of this space can be reduced by allowing the animals to live below your own home with a second-story setup.
2 acres of garden for all of your needs brings us to 17.5 acres/overall. We will include our necessities as listed above for wintering of horses, sheep, cows, and pigs (2/acre for our larger animals, 8/acre for smaller) and figure 6 sheep, 2 horses, 2 cows, and 6 pigs to let us sit at 3.5 acres. 14 acres left. Then our requirements for the family altogether... We'll figure a quarter of the overall for the animals as included in the family land, and the 2 acres for the supplemental garden. With our 4 kids and 2 adults we have 6.7 acres needed to maintain the family foodwise. Just for sake of not crushing everything we will make this also include all plant-based necessities like hemp for clothing. 7.3 acres overall 'available' for use.
Now this is just figuring that there is no fallow space at all, and no cycling. If we have a 'fallow field' of 1/3 of the plot (where we can place our animals) we bring ourselves to 4.6 available acres for the farm. This is again figuring absolutely perfect conditions and paying for shepherding and animal handling services, and crop tending assistance when you need to do the whole crop delivery/processing. You would also be paying for the use of the local herd for, ahem, fertilization of your fallow field.
Your acreage will produce around 20 bushels/acre. For fallow fields we will place the yield higher, and of course there will be a large variable due to seasonal issues, crop damage caused by hail, etc. Now the average rate per bushel of wheat in the medieval period (look at that Agriculture post above) would be about 40 ducats. A pound of ducats by weight figures them to count as 2.8ish ducats/GP, so we will count them as a simple Electrum piece (5 sp). That should balance out the rest of our completely WAGing on this thread

. 736 GP! Amazing!
Now how are we paying to get this all to harvest, processed, etc?
Hire 10 farmhands, a foreman, and carts to take the materials to and fro. You will need at least 3-5 days to process the harvest from scything, shearing, threshing, chaffing, and potting to your barrels into your cart. With a cart's
half-ton capacity per the 3.0 PHB (cannot remember the amount in 3.x but this seems to crop up for 3.0) you will need 6 (5.52) carts, each with a single horse to draw them. Then you also need teamsters for at least 4 of these (your wife and yourself have Ride to control the animals at a good enough level to not bust an axle, right?). Oh, you also need to supply some sort of ride or pay the mill's price to offload, process, and reload your wagons (probably double to triple your cost if your farmhand crew won't come along).
Let us say it takes a day's ride to get to your granary. Your teamsters need to be hired in town, come to you with their carts, load, and leave so they're going to be doing a total of 3 days to the granary each. Rental of carts costs a minimum of 3 cp per day (we'll use a Carriage cost) per person that could be carried by the cart. 1/2 ton of people with gear we'll put as 4 people, so that's 1 gp 2 sp/mile.
24 miles with 6 carts is 172 gp 8 sp. Your crew costs you 13 sp/day for your farmhands (7 gp 8 sp) and your teamsters run you 1.2 gp/day for 3 days (3.6 gp). So 184.2 gp for the cost to get everything to the mill.
Now you're dealing with grain merchants. Sale of products will be for half the overall price. You'll be paying for processing of course (let us make it cheap, so 1 sp/200 lbs, 2.76 GP.) 181.04 gp. You want to process your own grain needs while you're there of course, so add another 4 carts (3.6 gp) and your processing fees (2.1 gp). Then carting your own stuff back to the house (2.4 GP) and unloading of your materials by your crew (.65 gp).
172.29 gp. Not a bad haul for a year's worth of work. Now figure the average life of a solid gelding workhorse is going to be 5-10 years, so put aside 30 gp/year as 'maintenance' for your horses... You're also going to need to pay maintenance on your livestock in case of drought or loss to predators (I'll put a pig at about the cost of a dog, same for cattle and sheep just to save us the hassle) so put aside 1/10 of their value/year at 30 gp, and chickens at an overall 5 gp/year for maintenance of the coop and possible fox or other destroying the house. Most homes will have dogs for their value as guardians and can sell the pups so we'll put them at roughly 'even'.
The farmer also purchases a hundred arrows for hunting at 5 GP, bowstrings, two knives for farm work, and new farming implements (a light hammer and a warhammer for 7 GP for hitting posts, 2 light picks for mattocks at 8 gp, a set of masterwork tools for farming including an almanac and small tools, 50 GP) a new flint and steel (1 GP), 10 new jugs for his beer (3 sp), fifty pieces of chalk for teaching the children and marking temporary notes around the barn on animals and planting schedule (5 sp), and a set of artisan tools for his woodworking and the miss's baking (10 GP).
The overall benefit of all of that hard work for the family? 24 GP 99 SP. The next year the farmer sends his sons off to prentice, strong farm boys with good minds, while the wife begins to cook and use her skills. She has mastered the arts of handicrafts, and begins to use her Profession check and Craft checks to make candles and other sundries (soaps, lotions, etc.).
Over the same period she makes 286 GP. The husband makes around the same from his Carpentry. This cash can be reinvested into the farm and laborers. Paying off the costs of your laborers with this cash may help you to get better rates at market by bypassing the local mill. You become more self-sufficient and can learn the skill of brewing to convert some of your grain into sweet liquid gold. When the sons come back home the family can spend their savings on investing in a Smithy (500 GP) to be placed on their own property if given the right by their liege lord...
You get the picture.
Really most farmers who GO anywhere are going to be doing so from Profession checks and the use of corvee labor. That's why Commoners have the checks available to gain. While there will be those who never learn a trade and wander about, those who learned any sort of trade will produce others that will learn the trade or profession.
Slainte,
-Loonook.