how much should a mortgage modification cost

I think it's a waste of time, money and energy to use a "professional" service in trying to obtain a modification. Those claiming 100% success, or "guarantees" are merely in the business of taking advantage of people, in my opinion. I'm not trying to say that they're all illegitimate, but a preponderance is, and I'd suggest you try to go it on your own or use a HUD-approved, non-profit, free service. You'll do well by NACA, for example, in my opinion. That's only if you feel too intimidated or unsure about going at it on your own, of course.