poor gas mileage,key light blinking,engine shaky

It could be a burnt valve, but you must first get a diagnostic test run for the light that is on. This will help direct the technician in the direction of which tests to run. Find a shop that specializes in diagnostic work. Ask for recommendations.

A burned valve would mean you would need a cylinder head repair. To get a labor cost to remove the head, you can check the estimator for "Cylinder Head Gasket Replacement". Then you would have to add the replacement of valves, a head overhaul, or head replacement. Those are items too complex fot the Esitmator tool. But the head gasket part will get you started. Here's the link: https://repairpal.com/estimator/mazda/tribute I must say that you really should get a proper diagnosis before thinking it's a valve problem. There are so many other issues that could be occuring. Spark plug, coil, fuel injector, PCM interference... Any codes present?