Faucet repairs can be expensive for a couple reasons. One reason is not all manufacturers put their brand name in plain sight, or at all, on their product and there is not a one-size-fits-all part for any faucet out there. They all have proprietary parts made by the manufacturer or by approved manufacturers of older model faucet parts, but you have to know who made the faucet and know the model.
For common brands like Delta, Moen, Price Pfister, and Kohler, they all have the brand name stamped somewhere on their products. Once you know the brand name it is not so difficult to find the needed part. But that is not the case with many other brands, especially the more designer brands.
The other frustrating part is finding the part needed, especially if it is an older model faucet. Tracking down parts for these can take a lot of time with phone calls and searching online.
Another reason the cost is high for repairs is the replacement parts for some faucets cost more than the faucet did originally, especially if it is an older model. Many of the manufacturers do not keep many parts in stock for older faucets or they only make them a few times a year on their assembly line, thus the price is higher for those parts. We have waited up to six months for hot and cold stems for a 1990s American Standard bathtub faucet for this reason. Also, some manufactures discontinue certain parts for their faucets and there are no alternatives.
All these reasons are why it can be expensive to repair a faucet. The upshot is usually a faucet only needs repairing once every ten years or so, so simply replacing can be the best option.