My perspective on the G4 castles is that the bulk of the worth comes from the exclusive ponies- obviously that's why you bought them!
No idea on the canterlot one, but the Wedding Castle Id guess around $20 for no ponies. I know they can go around $50+ with the ponies and the outfitted ponies can go around $30+ themselves. My fiance found one complete in box with ponies from craigslist for $20, but that was obviously lucky!