This is a little late seeing as this has been opened for 8 months, but here we go!
The "Rooms" rules from Ultimate Campaign provide rough gold values and sizes for different types of terrain. We have 3 examples of rooms that could be viewed as purchasing land.
- Burial Grounds, 20-30 squares, costs 350 gp
- Farmland, 60-100 squares, costs 600 gp
- Rangelands, 300-500 squares, costs 2000 gp
So taking these rooms and dividing the maximum squares by the cost we can get a cost per 5ft. square. If we view theses as the average cost of buying land per 5 ft square, we can break it down to
- Uncleared Land (rangelands) 4 gp per 5ft. Square
- Cleared Land (Farmlands) 6 gp per 5ft. Square
- Cleared Enchanted land (Burial Grounds, has consecration effect) 11 gp 6 sp per
An Acre has 43,560 square feet, 25 square in a 5ft. square, so rounding up roughly 1743 5ft. Squares in it, by taking the cost per square at multiplying by 1743 we get roughly...
- Uncleared land, 6,972 gp per Acre
- Cleared Land, 10,458 gp per Acre
- Cleared Enchanted Land, 20,218 gp 8 sp per Acre
Now Its doubtful they are purchasing by the acre so its best to use the by the 5ft square price.
Now lets talk about some assumptions this makes
- The Farmland isn't more expensive due to being fertile land.
- The land is empty, raw land.
Obviously if the land has buildings already its probably required to purchase them and remodel them rather than demolish and start from scratch, or at least reclaim a portion of the buildings cost in materials if you do demolish. Additionally within the system it is unknown if the cost of buildings/rooms include the land, or if the earnings include the cost of leasing the land the building is on, aka paying rent for the land before you get any profit. If the earnings include leasing the land how much more profit to the buildings/rooms generate if you own the land rather than leasing. So these are decisions you must make as a GM.
Additionally the price can and should come at a premium for desired land/Buildings in exclusive/expensive areas so I would suggest using the above as a base and adjusting up from there. Hopefully this is helpful!
Edit: Thanks to Cellion for catching my math error.