So typically what are the total out-of-pocket fees for filing these requests including the bill from competent lawyer? How likely are they to succeed in invali

So typically what are the total out-of-pocket fees for filing these requests including the bill from competent lawyer? How likely are they to succeed in invalidating an issued patent? If done poorly, couldn't they have the potential to actually reinforce a lousy patent? Is this "cheap" course of action actually available if you're already being sued over some bogus patent you didn't know existed (or didn't choose to pay for the reexamination) in advance?