A faulty mass air flow (MAF) sensor could cause this trouble code to appear. MAF sensors are normally pretty expensive, around $150 - $200 for the sensor. Replacement however is relatively simple and should only take around 30 minutes (about $35 labor charge). To me, 400 for this repair is a bit too much, you should be able to get a cheaper estimate elsewhere. If you want to do it yourself, it just takes removing a few bolts and disconnecting an electrical connector to replace it.