Ok, the mass air flow sensor is located ON the air filter box and the part is only about $150 so this is not your issue.
It sounds like you are trying to describe a air fuel ratio sensor. There are 2 of these sensors, one in each exhaust manifold on either side of the engine. You probably have a code like p1135 stored here. No this would NOT be related to your last service.
Yes this price is fair to replace, the part is about $260 plus another $140 to diagnose and replace is right on target. This is also a common fail item on the 1mz-fe engine. I have replaced these with as little as 20,000 miles or as much as 180,000 miles. They go at any time.
Former Lexus Master technician
Master Certified with lexus, 6 years experience