Service Engine Soon came on recently

This previously occurred about three years ago. The trouble codes were read, and first the gas cap was replaced. After about three days the light came on again, and I returned to have them find a fault with a small canister in the emissions system that was faulty. That was replaced, the light reset, and that solved it. As I recall it cost around $400 parts and labor. The OEM gas cap cost $30. (I can't find the receipt!). After the problem was fixed I had to put 300 miles on it for it to pass MA inspection after the light was reset. My inspection is coming up at the end of October, and I want to avoid having to go through this again by waiting too long. The problem is my mechanics can't tell me how much it will cost for it to pass inspection. They mentioned an air leak in the gas filler pipe (very expensive to fix), but won't know until they go down the list that the trouble codes display. What are my options to fix this possibly expensive and elusive problem? I don't have the funds to buy a new vehicle at this time, and need transportation daily. Any ideas?