I guess I never really answered the original question.
I buy self-published books very rarely. The most I've spent is $5.
Theoretically I would pay the same price as something from a big publisher, but the self-published work would have to do more to prove itself, since it probably wasn't edited professionally (from what I've heard there are often a lot of different stages of editing that the publisher does, and I think that often shows in the finished work). There is also a perception, whether or not it's true, that many people self-publish because their work isn't good enough to be accepted by traditional publishers. So there would need to be things like an amazing sample chapter, a large number of almost universally good reviews, and/or a topic that I'm extremely interested in.
Although, to some extent, the same is true even of $2.99 or 99 cent books. At that price I might be more inclined to buy something that looks interesting impulsively, but it still has to be something that I think I'll like.