Skyroad
I think it would be even better to make it like real estate in the real world by allowing other mages to make offers on realms they want. Say I have a realm I bought for 20 and someone else wants it, they can make me an offer of 10 or 20 or 30 or whatever and it would be the law of supply and demand that would dictate the final cost. Just like investing in real life. It would make the potions more like real currency and would make the game more interesting.