What is a transformer in the heating and air conditioning unit? What is a ball park cost of transformer?

A transformer is a device that takes a certain voltage and reduces it to another voltage, in the case of your furnace or air conditioning system the transformer in the furnace changes the voltage to 24 volts for control of the componets in the furnace and air conditioning unit. The cost of the transformer might vary but will cost between $15 and $40 dollars not including labor.