depends on location
Keeps, you could place a fel keep right now if you look hard enough.
Fel Casltles are about 125-150 for decent locations (skarea brea, minoc bowl, etc)
less than 100 for a swamp
ice island between 100 and 120
Much much more for better spots like marble island, the spots to the right of vesper, etc.
Tram castles are cheaper (roughtly 100 mil, 80 mil for spots like the swamp or high spawn locations).
People may dispute these prices (higher and lower), however I have sold sevaral hundred castles in my day and I have had no problem getting quick deals done with these pricing guidelines.
I may have one for sale now if you are interested (would need to look at my lockdown situation first).