I'd say that a very great deal of it depends on your economy and location location location.
Lands within 3-5 miles of a town (walking distance) will go for the most, with the closer, the more expensive. Being along a major road might go for more, depending on how often the caravans and patrols decide to camp in (ie, trash) your wheat field or cut your fruit trees for firewood or repairing their wagons (or building pallisades).
You could buy one farm and rent it to a single tenant and that'd be fairly straight forward ("Heh, adventurer landlord? Just hold on long enough and the guy will vanish without a trace. Then it's mine!") But more than that would probably require having some kind of property manager. This is a 'Position of Trust', so I wouldn't allow them to hire this person in a day (seemingly at random) without possible dire consequences. This person could easily sell the property and and/or abscond with the rents.
For in-town property, it's back to that 'locationx3' thing. A great spot on the edge of the market square will go for a great deal more than an entire block in the slum.
It's all up to you. Sorry if that's vague, but read it this way - your question is as open as asking about owning property in the real world. Lots of variables. A small lot on the edge of Times Square can go for more than an entire ranch in South Dakota. And it would be an entirely different thing to develop, rent and/or manage.
One of the things I've always allowed PCs to do is purchase "great tracts of land"

of questionable value (because they're undeveloped, off the road and/or infested with monsters) for development as their future "baronies", ala OD&D. A great way for local lords to take in a bundle of coin with little risk.