I think Mojang should make a monthly price, a yearly price and maybe a hourly price so the player can adjust their server ontime and yearly price have discount... We should just wait for official announce.
See minecraft clones have unlimeted world some have texture packs, and some even have multiplayer servers. for free. but in the next update we should at least get 50 more blocks of world. But it shouldnt cost more than $5 or else thats a rip off. I havent got to test yet but i hope we still have local multipayer.
This is what mojang says about the PC REALMS: "The subscription price per server has not been set, but will likely be around $10-15 per month." Further down they say: "Minecraft Realms for Minecraft: Pocket Edition, will be available through an in-app purchase, though further details have not yet been announced."
(link: click here!)
As you can see, the pc realms will cost 10-15 dollars, but the price of the pocket edition realms has not yet been announced. Hold your horses and wait till mojang announces more details about the PE realms.
Besides that, a pc server is alot more demanding than a pe server. The world of a pe server is limited, and the fact that a mobile device can run minecraft says enough. They probably can run at least 10 pe servers on one 1 pc server.
I was also under the opinion that realms would cost up to $15 per server. I was informed that that would be the price per month, but I am not sure if it would be such a reasonable cost. I could believe something more along the lines of $5-$8. Then again, that's just me.
Considering what Gdrom said, it really should't cost a ton, but as blaffbellow pointed out, servers can be quite costly, so I would have to guess around $5-$6.