Fuel injectors.
Timing belt.
O-ring.
Struts.
Battery.
Sunroof.
Leather seat covers.
Power antenna
Overall, I love my Grand Prix. It has great handling and plenty of power. With almost 140000 miles on this car, I think the repairs that I've made over the years have been justified. To date it has new fuel injectors, struts, and new leather seat covers. I also had to make a repair to the sunroof, as it stopped working. The engine on this car has been well maintained all of its life, and runs strong, as well as the transmission. Do note, however, that the 3.4 DOHC engine is costly to work on, as you have to take it apart to do many things. For example, changing the o-ring around the oil pump required removing the heads, and timing belt, and ran very high in labor charges. If you take care of the engine, and know how the previous owners treated it, you shouldn't have too many problems. All parts wear out eventually though, even if it's a BMW or a Porsche.
The brakes on this car do need to be replaced more than the other cars at my house, of course, it's a lot of car to stop.
The key to this car is maintenance, rather than waiting for the car to tell you something is wrong by breaking down. It may cost money to maintain, but again, the labor on these cars is expensive at GM.
There is no need to replace your Fuel Injectors just because your mechanic suggests it. All you need to do is unbolt your Intake Manifold, and WD40 The Injectors at the Nozzles. Of course you will still have the expense of a new Intake Gasket, but that sure is cheaper then what your mechanic charged you. Try and find yourself a good local shade tree mechanic.