On an eight-year-old car with that kind of mileage, things are going to break. On a vehicle like a hybrid, these things can be expensive. These are realities you accept when you buy a hybrid. Fortunately, hybrids help you save money on gas, but you know components like the battery pack and the electric power drive will eventually fail, so for everyone considering a hybrid, if you are going to buy one, budget for repairs by setting aside some of the money you are saving on fuel.
@jen207, it's not necessarily crazy to spend $4,200 on an 8-year-old car if the car is in otherwise good shape, it's paid for, and you plan to keep it for a long time.
ok4450 is right. You wouldn't bring your own food to a restaurant and ask them to cook it, and you wouldn't bring your own harvested kidney to a hospital and ask for a transplant. Your mechanic would open up a can of worms in liability and if you happen to buy a bad used part, your mechanic is the one who could suffer by possibly damaging his reputation, losing out on the time and money he put into the project, and leaving you dissatisfied.
Bite the bullet and get this car fixed the right way. If you take good care of this car, it will last a long time, but when you plan your financial future, budget for repairs. Expensive repairs are a part of owning a car.