My "Service Engine" light is on and the mechanic says my mass airflow sensor is not working. Should I replace it for $450+? What does a mass airflow sensor do and what will happen if I do not replace it? This is for a 2003 BMW Z4

you can test them yourself. start your car and take a screwdriver handle and tap on the sensor, if it stumbles and dies, its bad.

I would first try cleaning it. If the maf sensor is "not working" your engine will not run. I think you need a better "mekanik". The check engine lite is an indicator of a problem not necessarily an indication that a sensor is bad. Get a second opinion.