Idle speed control motor 85 Ford Tempo

Suddenly the car accelerated wildly and my mechanic said it is the idle speed control motor or called the air control actuator. I am having a part time mechanic replace it for me. How much time does this job usually take and what does it usually cost please? What would cause this to go out? He replaced the thermostat 2 weeks ago and it looked smaller than the one he replaced althou that is the part they ordered and arrived. He then put some blue gunk in and said it was for making a gasket. Would that have caused this problem? He also replaced the fuel filter. Any additional thoughts would help as I am on a tight budget and also had the water pump and a couple hoses replaced so now this. Thank you.