I almost always replace the capacitor on a motor when I install a new one. Capacitors have a life span like batteries do. An old capacitor is likely to fail when you continue to use it on a new motor. For the $20 or so it costs to install one, I'd do it every time. If a repair tech / AC company wants $600 to do it, find another AC company. Tell the one saying $600 to shove their bullshit price tag up their ass.
Even if I were doing it for a living, I wouldn't mark the capacitor up past 100% of my cost at the most. That'd be maybe... maybe at most me charging $40 bucks for it and the install, on it's own doing nothing else--as a full time contractor doing this stuff--would be $50 to $100 depending on the market. That's at the very most 25% of what that fuckstick was charging.
In fact, I had the start capacitor on my AC compressor blow. I opened the unit and trouble shot that. But it was a cold drizzly day (using it as heat pump) so I called the AC guy I sometimes use. He came over and changed the cap, cleaning up the wiring and such, like I'd have done. He charged me $75, and I gave him a $100 bill to be nice. I found that a reasonable price so I didn't have to get on my roof and pull the cap, then drive to a parts store and get a replacement, then get back on the roof, and clean things up and put the new cap in. That would have been several hours of my time. Better to just pay $100 and be done with it.
Never replace a circuit board unless it doesn't work. If it ain't dead, it's not worth changing it out.
The way most larger service companies work is the tech gets $50 to $75 a billable hour pay. He gets ZERO for driving to a service call. There is a trip charge that goes to the company for wear and tear on their truck. The tech sells each part used at the company rate. He gets more pay for installing more parts, and for having more billable time. Each operation he performs has a book rate the company pays him out for. The company may pay him a cut of the profit off parts too.
Thus, the more he sells you the more labor he gets paid to do. He's going to try and tell you you need all sorts of stuff you don't need.