Auto air parts can very widely in price, depending on where you shop. There are several reasons for this, but the most common thing that raises prices is the number of middlemen involved. To get an automobile air conditioning system from the factory to the parts shop on the corner, there's at least one middleman involved, and his cut adds to the price.
This is why the best way to buy an automobile air compressor or other A/C part is by going factory direct. Look for parts vendors who leave out the middleman, and bring you the factory direct price on their parts. This will save you a great deal of money on auto air parts, which is money you can sink back into your car.
Many places might claim they sell their parts factory direct, but do they really? The proof is in the pudding: look at the price of the parts. If you do some comparison shopping and find that "factory direct" parts are as expensive as they are at a dealership, those parts aren't really factory direct.
Of course, just as important as the price of your A/C parts is the quality. Whether you're fixing or simply upgrading your air conditioning system, you don't want to use parts that are in any way sub-par. Look for a parts shop that stands behind its products, and brings you the highest quality in A/C components.