Anything that requires large things to come out of the car or a lot of surgery to get to. A water pump is a very cheap part, but on vehicles with timing belt-driven pumps, there’s so much disassembly involved that you could often, literally, spend less on a week long Caribbean cruise.
It’s especially fun when something breaks behind the dashboard, particularly in modern cars where getting the dashboard out involves hours of work to be done before the mechanic even starts working on the thing that’s actually broken.
But the most expensive repair is probably when the mechanic replaces parts almost at random because a computer told them to or because he assumes he knows what the problem is without actually bothering to test his hypothesis, and ends up taking 5 or more tries to get the car working right. That scenario is actually what got me started working on my own cars, because a mechanic (and I was even doing the right thing and going to a local, independent shop!) spent well over $2,000 of my money throwing parts at a stalling engine when all he had to do was replace a $100 fuel injector.