Have Blizzard said Classic servers will have their own sub fee? If not, I imagine it will just be some special servers anyone with a sub to current WoW can access.
Will a 100k people play regularly on Classic servers? I have my doubts. There's also the matter of how long people will stay around in a game that's in a stasis. If PvE is your thing, then you are pretty much on the top of the mountain after defeating KT.