I charged 55 for an acoustic/gibson style nut. That includes new bone nut, setup, strings, and general cleaning. Depending on how kind the person was, I might do quite a bit more just for the hell of it.
65 for standard Fender style nuts, 75 for radiused bottom nuts.
It's a nut. You can stand there while he does it. If he screws up, don't pay him, and have someone else put it on. At least you'll have given him a chance. Maybe he's just doing it for suplemental income, and since he doesn't have the overhead of a shop, he can do things cheaper. Don't ever count out the little guy.
I say give him a chance. The worst he can do is screw up a $5 nut.