1999 F350 175,000. The truck on startup is intermittently spewing white smoke. My mechanic, a large reputable establishment, replaced #5 fuel injector at a cost of $800. Two starts later I get a service engine light. Codes have not been run yet, but mechanic previously said all injectors should be replaced. I was searching the Internet and saw two references that said the price should be about four to $500/per. Is $800 per fuel injector a fair price? Help!