Since no one is replying, I would wager 15-25mil (siege gold). I think I paid 5mil for my Keep a few years ago, but I think the last one I saw went for 15. Plenty of 18x18 spots around, but anything bigger your on you own. Since people can have a house on Siege in addition to their Prodo shard house many people just placed something on Siege and then forgot about it ; ).
Btw this is Meat (popsicle). You playing Siege now? Been here a few years now in semi-retirement mode ; )