The grooming places I've been to, the groomers either work for hourly pay (usually around $6.50/hr which is very low - fast food even makes more) or if they get enough hours they commission out - 30 - 40%. Also the places I go, they have to have their own tools, etc. just like a hair dresser. The shops don't provide them with anything except maybe shampoo. So it can get very expensive. If they're a good groomer they'll make sure their tools are sharp, etc. That all really adds up.
The only reason I know some of the things is because I've thought about getting into grooming. Even if I did it part-time it would be very expensive to start up at a shop anywhere for little pay. Now I'm scared even more hearing that most people don't tip (I didn't tip at 1st either because I didn't know).
I'll tell you though, since I've started tipping (and I tip my groomer any where from 16 - 33%, usually 33% - $30 per dog and I try to tip $10, but have done as low as $5 when I've been short) I've noticed a difference in my service. I have an easier time getting fit in for those hard to get appointments, they use better products on them (without the extra charge), she will take the extra time if needed to calm them down (my oldest doesn't like the dryers so she hand drys her, my youngest is scared of strangers and she makes sure she's not exposed to anyone except her), she gives me discounted pricing and usually does little extras to them.