I don't really know the ins and outs of how. I've never been sued by patent trolls. I've been threatened, just responded with some of the evidence I have and a, "Are you sure you really want to do this?" and I never hear from them again.
When I've helped defend other companies (two of which are among the largest/most prominent in the games industry), I've just provided expert witness testimony as to the existence of prior art. I've never had to testify about it in court, so I'm fairly sure that each time the trolls dropped it before going to court.
As to how much it costs - no idea. They had pricey lawyers though, so there's certainly some cost involved. I suppose it depends on how big the balls on the troll are, or perhaps how easily they think the defendant will be intimidated into just settling.