what is a fair price for a spark plug & spark plug wire change?

https://repairpal.com/estimator/hyundai/santa+fe The Estimator tool (link above) will get you good pricing averages in your area for the spark plug replacement. To replace the wires at the same time, figure an additional $220 or so parts and labor. (wires are not in the Estimator tool). So, his price is not a bad one, referencing the information. Plugs and wires are an excellent proactive place to start. However, it may not get your economy back to expected levels. Make sure your air filter is clean and your throttle body is clean (sounds like it may have been cleaned when your injector cleaning was done). Make sure you have had your fuel trims checked -- it may be here that more fuel is seen as necessary. Good luck!

It sounds more like you may have a lazy oxygen sensor. Is the check engine light on?