Expense is high because #1 it's an esoteric "specialty", #2 because modern pump test equipment is very expensive, and #3 much of the repair info and tools are not easily available to the general public - so they know they've got you by the n*ts.
Many pumps called "rebuilt" are not. Many are cleaned up, resealed, recalibrated and many high-hour metal parts used over again, as-is. NO renewal process for some. It would not be unusual to pay $700 for a Stanadyne rotary pump on a Deere that received $50 worth of new parts and 2-4 hours labor.