Sticking Thermostat

My thermostat has been sticking closed for a few months now so I took it to a gas station to get a new one installed. Everything was fine for about 3 miles. the temperature needle climbed steadily so i pulled it over and the coolant was boiling. i looked at the thermostat housing and the screws were completely gouged out. so now i want to take it to a real shop. does anybody have any idea of how much it would be to get the screws removed (they're completely round now)and have the thermostat actually installed?