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.