$30 per reg labor charge is excessive ONLY if a customer brings in his gear clean and corrosion free and completely ready for just a parts swap. I've been doing this for a long time, and "20 minutes per reg" only happens in the DEMA clinic with loose new regs.
About 80% of the regs I service need a detergent or phosphoric acid-based dip because of the verdigris corrosion (what owner thinks he got good service when he gets his green-flecked reg back that way?), then maybe a few minutes in the ultrasonic, then pressure air drying and hand cleaning the dried excess Christolube from the last shop.
Many pistons get a little hand polish with 6000 grit Micromesh at the knife edge due to sand blasting pockmarks where high pressure air enters from the tank.
Now clean the sand out of the second stage between the barrel and the adjustment knob. Inspect the diaphragm for holes. Don't forget to disassemble the exhaust for the occasional cockroach nibbled exhaust valve.
Torque it all to spec and tune it perfectly. Pressure test it in a big sink to test for any leaks (or too many bubbles on that HP hose that you didn't want to spend $40 to replace last year). Dry it again. Package it up with photos of the problems you fixed to justify those expensive replacement parts (yes, I'll agree with the other posters about the cost of kits), and then fill out the bill.
Twenty minutes? Never.
I'm certified by over a dozen manufacturers and LOVE doing it right! But I only do it for my friends, or when the shop really needs extra help, because you can't survive on what you earn if the shop charges $25 per reg retail.
I started for the same reason most of you did: at $30/reg, I was so disgusted by the quality of the service I got, that I got trained to do it myself. But you can't make money at those rates if you do it perfectly, unless you're doing it at scale, with six guys all working full time in a big shop.
As others have noted, the key is finding that rare good shop. You can argue about the parts cost, but the labor charge is a steal.
My 2 cents.