Just FYI since I saw your post on uoforums castles usually go for 250-300 mil
I recently purchased one in a good location for 300M in fel.
Keeps come up pretty often. I suggest getting in touch with Giggles from Blunt Logic. She's a real pro at placing them and often has a keep for sale on some facet somewhere. You can find her here on Stratics or in game (weekends are tough though)
She's actually the one who brokered my castle deal. One of the most trustworthy people in the game IMO.