02 S model coolant pressure problem
 

my 02 S model has 120 k and runs well. I comute 100 miles rt 3 times a week. 2 months ago the temp gauge pegged light on and I pulled over in bout 3 sec and shut the car off, fan ran for a minute and went off. came back that day refilled coolant and drove 50 miles home. replaced the expansion chamber and all is well untill Sat am and the same thing happened. I had coolant & h20 in the car and it ran out off the bottom of the chamber. towed to dealer and they say best case: new head gasket worst case: new head = rebuilt motor. GT ranges between $1300-5000 much saddness & WTF? does this seem reasonable?