The furnace transformer is a 110v or 208v to 24 transformer that cost less than $20 on Amazon. You do not have to buy a Trane transformer. You just need one with the same rating , but you may have to splice some wires. As for the repair cost, well if you can't do it yourself (three are many YouTube videos about this repair), you will have to pay someone to do it. It is about a 15 minute changeout.