1995 Pontiac Grand Prix Overheating engine

For about a week my car was running very poorly. I knew I had a misfire and some days it would not start without a jump from another car. I was driving it home from work and the low coolant light came on. I stopped at a local auto-parts store and thew abput 2l of coolant in the reserve tank and drove it home. The next day the car would not start so I got a jump from another car and I drove it about a half a mile and the temperature shot up to 125 degrees. I pulled into a parking lot and as soon as the car started to idle the themostat dropped down to aboyt 100 degrees which is where it normally runs at. I took it to my mechanic and he informed be the "Center Dome" seal on the engine is gone (what he describes is something close to the head gasket). From what he tells me it could cost me close to 800 dollars to repair. Now this does not make sense to me as I had to replace a head gasket on 3 cars and it never cost me more than 450 dollars. My question is this: Do you think it is this "Center Dome" and is ti worth 800 dollars to be fixed or am I getting ripped off.