The cheapest way to invalidate a patent
The cheapest way to invalidate a patent is to just watch it and let it lapse.
Half of all US patents will lapse at the 12 year mark, and the number will be significantly higher the longer you leave it. The numbers are higher in other jurisdictions where maintenance fees are payable each year.
So next time you identify a patent that looks to be of concern during a freedom to operate exercise, think carefully about the actual future date that you will be wanting to launch.
It may well be that you can wait it out a while before deciding whether to ‘invent around’ or develop an invalidity position for the patent. (And, sure, if you have the funds now, then go ahead and develop an invalidity position as well – but that’s not everyone at the moment…)