I've touched on this topic before, but now want to address it head-on.
Times have changed in the machinery repair business. In the old days, it was common to rebuild alternators, starters, hydraulic motors and other bolt-on components. Today, I can't remember the last time I rebuilt a starter or alternator. I rebuilt a hydraulic motor last year, but it took a lot of talking from the customer to get me to do it.
That's because when I compare the total cost of my labor, plus the repair parts needed to rebuild things, it's often as expensive as a factory rebuilt, or brand new starter, alternator, hydraulic motor, etc. It's tough to do a good rebuild in less than a couple hours. With shop rates pushing $100/hour in many areas, that makes it tough for a mechanic to rebuild things cheaper than simply installing a new unit.
Even if the new unit is several hundred dollars more expensive than rebuilding the failed unit, I consider it a distinct advantage that the new unit has all new parts, and was rebuilt and reassembled to factory specs. If I rebuild it, some of the housings, shafts and other components will be re-used, so there's risk that microscopic wear to those parts will cause the rebuild to have a shorter life than desired.
I'm not against rebuilding. Rebuilding is always an option. A good mechanic should know the pros and cons, the risks and expenses, and be able to give the customer a fair appraisal of whether or not it's better to rebuild or replace. That hydraulic motor I rebuilt last year? It's still running, and the customer saved money by rebuilding instead of replacing. It was his money, his decision, and I'm glad it worked out for him.
So, if your mechanic suggests replacing rather than repairing some component on your farm equipment, don't assume he's trying to gouge you. Talk it over with him. In the long run, spending more money to replace a component might be cheaper than saving a few dollars by having it rebuilt.
Like I said, it's your money, so it's your decision.