The great thing about medical billing advocates is that many of them work on contingency. You don't pay them a thing unless they negotiate a discount for you. That's how confident they are that they will, indeed, find errors. If they do find errors, typically you then pay them a commission of 15 to 35 percent of the savings found.
Here are some resources to help you find a medical billing advocate.
CareCounsel (A service available through your employer only.)