What is the cost of repairing an electrical short in a car?

Unfortunately the question is "how long is a piece of string?"

It depends on how easy it will be to find the location of the short, and how accessible that location turns out to be. It might take 20 mins to trace it and 5 mins to fix, or it might take 3 hours to trace, and 4 hours to strip out the dash to get to it.