It feels like you never looked at https://www.uspto.gov/web/offices/pac/mpep/s2215.html and it might be worth giving that a quick read before pulling a number out of thin air.

Yes, it costs money to get a patent invalidated based on prior art. But: an amount that even a single person who stands to actually gain from having a patent overturned should have no problem with. You're asking the USPO to spend time redoing work, literally halting any other patent work they could be doing instead. So it's not a trivial amount, but it's also hardly a prohibitive amount if you actually want a specific patent revoked.