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.

You're a good person. I hope a lawyer with your character contacts you about pursuing sanctions for vexatious litigation against these guys.

At least invalidating the patent.

How come if they have sued and lost based on prior art, the patent is already invalid?

They probably have never lost in court but rather settled or dropped the cases before they get to court.

Sorry you got rate-limited in the comments here; it's a restriction that new accounts are subject to because of past abuses. I've marked your account legit so it won't happen again. Welcome to HN. We hope you'll stick around!

The following is not legal advice and should not be construed as such. Short answer is that it really depends on your strategy and the rate of your firm. For example, a colleague of mine works at a top tier firm. They were hired to defend a major auto maker against a troll. They took the case to trial, won and then successfully countersued for repayment of fees. The whole thing took a couple years (patent cases move at a glacial pace), and the lawyers ended up billing the car company for several million dollars. I also don’t know that they were ever able to successfully collect on the countersuit judgement.

Another strategy is to try and invalidate the patent via the USPTO before it goes to trial. This often does work with bogus patents, but it’s still not cheap. I have heard the ballpark figure is $100k.

This is an effective threat because it means they lose the whole patent, not just the case against you.

$100k for a complete end-to-end IPR is at the bottom of the market. You can find it, but $2-300k is more the norm.