What most people fail to realise when they look at "farmhouse" accomodation is that we Italians have a slightly diferent meaning for the word. It usually refers to 4 to 5 star luxury resorts with built in farm-like activities.
The high prices are sort of justified by what you get and they also only operate during the busy months.
I know a small wine farm close to Montespertoli (where i live) that i've been told gives somewhat reasonable prices, altough the rooms are a bit on the "rustic" "old fashion" side, certainly not luxurious.
I sent you a message with contact information as i am not sure i am allowed to share it here. Otherwise my email should also be in the signature.
As far as visiting wineries/castles, while it is true that staying anywhere in the Chianti wine region, including places like Greve, you will be within a small distance from alot of them, actually getting to these places might not be as easy as it seems unless you are willing to bike or run ALOT.
While public transportation sort of exists, it will mostly take you beetween cities and you will have to actually get to the wineries from there. Some of these places can be quite out of the way.
I am inclined to say that a large group bus type tour might be harder to find unless you start of in a big city (like Florence or Siena), but i may be wrong.
I would advise either renting a car, hiring a private driver or booking private tours. Altough the big group option i mentioned earliar might be cheaper.
Hope this helped.