How much does electrical wiring cost on impala 03?

i have a 03 impala 3.4 v6 about a month it overheated and we got the radiator fixed..2 weeks later we found out also the head gaskets was busted so we also got that fixed. then we got the car back and the temp still kept going up took it bac andsaid it was the thermo so they changed it. then they said it was the heater censor so i had it back for days and it was good but yesterday the temp went up and the coolant hot light came on..i was pissed by now! so this time i took it to sears auto they looked at it and said the car was not overheated the temp was actually 180 but the gauges were reading it wrong could be a faulty censor or electrical issue WHAT COULD IT BE? and how much? please help