I'd price out what the supplies cost.

10x - Solid #8 or #6 x 3 wires per outlet @ 20 feet per wire
10x - 50 amp breakers
150-175 feet of EMT
10x - 2 gang electrical boxes.


That's the material list or close?  Price it out at Home Depot.  I think you're way over 500 in materials.  But not 1000s off..

Then you'll know what you're paying for his time.

Also, get at least 3 quotes.