Gas leak

I understand your frustration but you're asking us to second guess a mechanic who has examined and diagnosed the car, whereas none of us can. It sounds as though the fuel line is leaking, not sure if your car has a throttle body or is EFI. I've worked on various fuel line and fuel rail assemblies and can state that they can be expensive parts. As a DIYer I can turn to the salvage yards to find parts but paying for these as new OEM parts from either a dealer or parts store is not cheap, and many times they will only sell as assemblies. Ask the mechanic to break down his quote, have him show you the part in question, what is wrong with it, and where he is sourcing it from. Ask if they can source a used part from a salvage yard to keep costs down.