That way, not everybody is hopping on the same classes as soon as the expansion releases.

It also ensures that the only people that can play the new classes are those who are supporting the game heavily.

Not to mention the fact that when you pay for a class, you are more likely to spend time playing it and learning it.

$50-$60 a class should be fine.

Sell a good chunk of those, and the voice acting for the story quest is covered.

Make them OP to make more sales. But like, not too OP, or people will whine.