tulle is the 'fix' of choice for many archival quilt collections. i was taught at the winterther museum that the damaged areas only were covered in matching tulle and handstitched with tiny, close invisible stitches to the firmest parts of the surrounding fabric. but you didn't say that the front of the quilt was tearing or shredding. is that part okay? am i correct that only the back needs replacement?
if the back and the batting are the only parts that need replacement, then your price is very fair if they are supplying the fabric. if the front also needs work, then you have to adjust the price before they accept it.