So, if I would create a System that allows players to have bastions/establishments from scratch, I would do it that way:
First of all, everything is Gold and Time based. No additional meta currencies.
Point 1: General building costs based on the material used.
To build Walls with roof, it cost per 5 feet of Wall (1 Hireling, you can hire more, Hireling to fasten the process, a second hireling halves the time needed, but the cost increases by 1,5)
- Wood: 1GP, one day (AC 10, 5 HP, damage threshold 5)
- Stone: 10 GP, 5 Days (AC 12, 20 HP, damage threshold 15)
- Dig into a Mountain/into Stone (5 square feet): 30 GP/15 Days
- Underground-Facilities/Cellar (5 square feet):
- Soft Ground/Clay: 10 GP/5 days (max. 1 Level deep)
- Hard Ground/Stone Ground: 30 GP/15 Days
- Upper floors:
Every floor above the ground floor increases the costs and time by half. So the second floor costs 1,5x, the third floor 2x, the fourth floor 2,5 times and so on.
Wooden structures only support a ground floor and one upper floor. Stone structures can support several upper levels.
If you need to drain water (Swamp-Area, by the sea) or have other complications, time and Money is doubled.
So now you have a simple system to build a shell for your buildings. The numbers are fast and loose right now, but I think they are in the right ballpark.
You can build a 10x10 Shed out of wood for 8 Gold in 8 days.
Out of Stone a 10x10 hut costs 80 Gold and it takes 40 days.
Playe characters can also reduce costs if the work themselves (like halve the cost for wood, reduce 1/4 of cost with stone).
If players want to buy a building, you can look at what it would cost to build that building and double that. There should also be guidlines on when to give buildings as quest rewards.
Now, after having the shell of your building, you can stock it.
You can put everything you want inside it.
To make that easy, there will be sets, for example for mundane rooms:
Bedroom-Set:
- Squalid (requires at least a 5 square feet room) - a straw mattress, a scratchy blanket, and a wobbly wooden chair - 2 sp
- Poor(at least 10 square feet) - a wooden single bed frame with a straw mattress, blanket and pillkw, a wooden chair, a bedside table - 1 GP
- Modest(15 Square feet) - a wooden single bed frame with a spring mattress, a warm blanket and comfortable pillow, bedside table, chair, dresser - 5 GP
- Comfortable(20 Square Feet) - a wooden bed double bed frame with a spring mattress, warm duvet and blankets, pillows, beside table, chair, dresser, wardrobe - 15 GP
- Wealthy(25 Square feet) - a comfortable queen size bed with spring mattress, nice duvet, blankets, pillows, bedside table, chair, dresser, wardrobe, dressing table - 30 GP
- Aristocratic (30 Square feet) - ... (Go crazy?) - 100 GP
So you have an upgrade path. I would also tie some resting benefits on the quality of the Bedroom, but I don't know what yet (which would work with gritty realism rules, like, in Squalid conditions you rest time doubles, in comfortable it halves and in Wealthy/Aristocratic you are so rereshed you start the day with inspiration).
So we would have such Price Lists for the most common rooms like the privy, a kitchen, dinning room and so on.
Then we get to the special rooms, like the pub.
Pub-Set:
Squalid (Minimum: 20 square feet, 1 unskilled hireling) - a wobbly wooden counter, 3 wobbly chairs on the Brink of braking, a shelf for Drinks - buying cost 1 GP, upkeep Cost 3 SP per day (Labor and Drinks, can change depending in what drinksnyou wanna serve and how many people younwanna hire) - makes 1d4+1 sp per day (can change depending on the drinks you sell, special occasions and other factors).
Poor (Minimum: 30 square feet, 2 unskilled hirelings) - 10 GP buy - 1 GP upkeep, 2d10 SP per Day
Modest 100 gp
Comfortable 500 gp
Wealthy 1000 gp
Aristocratic 5000 GP
One can also summarise upkeep and profits into weeks (tendays) or month (30 days) to cover longer periods of time.
So for a ten week the upkeep Cost of the Squalid Pub is 30 SP, the income is 10d4+10 SP.
For a month upkeep would be 90 sp and income 30d4+30SP.
And than add some special abilities. Like with the Squalid one, you can hear rumors about things going on with the poor people or gain favor with the poor and criminal elements of the city by giving out rounds, with the poor one you can gain favor with the working class and so on.
And now do that for all the special rooms mentioned in the Bastion UA and adapt that accordingly.
Now players can decided, if they want to have 10 pubs in the city or build a big keep together with a gambling Hall, pub and arcane study or if they want to separate their pub establishments from their private rooms and so on.
So, what I actually just want and need are building costs, upkeep and buying costs for single bulsings/rooms/establishments, descriptions of what the rooms can do mechanically and upgrade paths, gated by Gold cost and building time and also special things that can happen to the establishment/building based in the Typ (like Brawl Fight breaks out in the Pub, giant rats find their way into the cellar ect.).
No bastion Points or turns or ant of that stuff.