Expensive parts usually belong to expensive vehicles or simply cost a lot to make. Older car parts are hard to find and the scarcity drives up the price.
These parts cost a lot due to who makes them. If they need shipped from overseas, due to a foreign brand, then the price will grow more. Mercedes has pricier parts due to the fact that they are a high end company that has to import their parts and use better parts with tighter standards. Remember, the Sum is greater than the whole.