What is the average cost to repair a Tractor Trailer?

There's no such thing as an average cost because it depends on a few things, like what engine you have (Detroits and Mercedes-Benz are the cheapest, Cats, Volvos and Macks are really expensive, and Cummins are somewhere in between) and what's wrong with the truck.

I had a starter go out on my Cummins ISX. Cummins used what turns out to be the most common starter in the world on it, and they're reasonably inexpensive at about $400, but the whole repair cost over $1000 because it had to be made at a rest area.