Is $10,000 to replace a leaking EPDM membrane under a deck over a garage a fair price?

I'm dealing with a leak in my deck that's built over my garage. My deck has wood boards on top of an EPDM rubber waterproof membrane. Part of the deck rotted. We removed the damaged wood and cut out part of the EPDM to patch it. We had an individual contractor do the job. He fixed the EPDM with a EPDM tape and Lapel sealant and it's still leaking! I want to repair it properly so it lasts through harsh winters with very cold temperatures and heavy snow.