Repairs to a 1999 Pontiac GrandAm

I have a 1999 Pontiac Grandam. It started shaking very badly and making a very bad rattling noise. It drives, but it is rough in driving. My mechanic states it sounds like a misfire. After a diagnostic test, he says that it is the plugs, coils, and the housing unit is burned. The estimate for repair work is roughly $285.00. Is this a fair estimate, and is this type of job different than a basic tune-up? Thanks in advance.
-Needing Assistance