From what I read when the minions came out in another region, the minions are a dollar or two each. So, expect a slightly higher price for Fearless.
But I don't think Fearless would be expensive. Think about it: if you made very character and stage in the game as a dlc, would it add up to the original price of the game? Yes it would.
The stage should be free just like the characters for a limited time. Why give us new stuff, say it's gonna be free for a short time, but then not make it ALL free. They're gonna put a price tag on the characters eventually, so why not do the same with the stage? It'd be much less of a headache.