I agree with Kirthag on her price for 18x18s.
Keeps and Castles are a bigger area and more of a pain for dyed colors (non-reward, non-abyssal cloth) I'd say 10-15m for a keep and 20-25m for a castle.
That being said, I do feel I have to give the fair warning that if someone reports the extra levels of goza mats because it "impedes them" for whatever reason, you do run the risk of a GM or advisor deleting it. I've seen this happen a few times over the course of the years - especially if someone places a house and it blocks them from seeing stuff in their home, so just know if someone decides to be a jerk, you run the risk of losing it all.