engine light was on

You aren't giving me much to go on. Do you know what the trouble code was? If the check engine light was on there is code(s) stored which would tell me a lot. Was there a valve replaced or just the gascap? If some emission or engine control part was replaced and it fixed it, then $360 is very likely a fair price. For a diagnosis and only a gascap (did it fix it???) $360 sounds very high. A basic diagnosis is about $100 and a gascap is maybe $20. if the gascap was tested and leaked, you needed a new one. If not, I wouldn't have sold one.