I might be wrong but the sensor should be about 60$. For all the time messing around with other possibles I usually just start with the sensor. If that is not it then vacuum and injector testing. Depends on how you value time. The O2 sensor can also be checked by a real time scan tool. It will show the actual readings real time under load and idle. Since I have one it is cheap for me. To have a shop do it will start at 60$.