According to a recent (1/2/14) Eleventh Circuit Court of Appeals ruling that the violation depends upon the langauge in the contract signed. In this case the court found that there was a violation of Fair Debt Collection Practices Act (FDCPA) when Franklin Collection Service, Inc. charged a consumer a collection fee that was based on a percentage of the debt balance rather than actual costs. The found that 15 U.S.C. §1692f. Unfair practices was violated.
See the original ruling in Bradley v. Franklin Collection Service.
It appears that the language that was in question concerning illegal collection fees was: