Take a look at an aviation parts house catalog such as this one from Aircraft Spruce. For example, look at the prices for oil filters. These filters are little different than the ones you can buy at WalMart for a couple dollars. However, when you factor in certification costs and limited production, it isn't hard to see why they're so expensive. About the only inexpensive thing that goes into an airplane is the air in the tires, and that's only because the FAA hasn't found a way to make people use certified air.

Now, think about the parts that go into the Shuttle. Few of them are available off-the-shelf. Since there's only 3 Shuttles left in the fleet, many of these parts have to be custom made and there's no way to lower the costs, making them very expensive.

Remember the stories from the 1980s about $300 hammers? The same thing applied back then. Some bonehead ordered a special hammer that ended up having to be hand made. Of course they were expensive.