Expensive car repairs can run into the thousands of dollars if you have to go under the hood. Your engine, your transmission and the related parts to both those items can cost you thousands. Though, that being said, it really depends on what car you are driving and what model it is. If you own a Ferrari, for example, it doesn’t matter what part you look at, it’s going to be expensive. Like all things, it depends on what the product is that you are going to have to repair. A vehicle that doesn’t have a computer is usually cheaper to repair than one with a computer. Computers that go wrong are always expensive. As to diagnose the problem, you would need to know how it worked. Most of the computers on board are sealed units and need to be totally replaced.
For more info, see 10 Most Common Repairs done at Car Repair Shops