It sounds to me like the best thing for you to do is just go with what seems fair and logical.
Like start with a percentile roll to see if population grows or shrinks. Allow the player to earn modifiers to this roll based on efforts they put into it. Such as active recruitment drives, cash/land/housing incentives, etc...
I would still not allow them to increase the population more than 200% in a given year (meaning no more than double it). Also, the previous posters are correct, investing money into such lands is a very wise thing to do, because with time and mother nature being on your side, PC land owners can make a lot of gold, even with 40% of it being paid to their lord as taxes.
The cool thing is, powerful PC's can negotiate as a reward from their lord "X" number of years being at a lower tax rate, even zero if the service is worth it to the given lord.
I wouldn't roll for such things. I usually "know" enough about the lord, their issues, their costs, etc... to know whether or not I would offer the PC's such a reward if I actually were the lord. Then I enter into real life like negotiations with the PC's based upon these things I "know" about the lord in question. No rolling, just me and the players making decisions based on what we know about the characters involved.
When the occasional thing comes up that we do not know enough about, I revert to a percentile roll, and giving a 50/50 chance is usually good enough, but again, I follow my instincts about such things, and modify the result needed up and down as I think fits best.
As an example, the "Magical Medievel Society: Western Europe", by Expeditious Retreat Press, recommended by a poster up above, does everything based upon the D&D assumptions of value. So, as a result, one of my players has a PC who has 100 square miles of very profitable land for his Barony. By the same token, based on the wealth of his lands, I have a much better sense of how disgustingly rich the other Barons, Counts, Dukes, common land owners, and the Empress are as well.
So now the whole economy makes much better sense to me since I understand the basic scale of it so much better.
If you ever decide to get into more economic details, let me know, because there are several other good PDF's and such out there.
Also take a look at the following:
Medieval Demographics Made Easy
Medieval Kingdom Generator
http://citygen.crystalballsoft.com/
and
Medieval Sourcebook: Medieval Price List
These can all help you figure things out and be more comfortable with your decision making.