If he is doing a layer of primer and then powder coat it'll be a bit more expensive and a much stronger bond. But I think powder right on the metal is fine. And no, its not that expensive. I'd suggest negotiating somewhere else. Not sure how i would feel about a guy doing work for me if he tried to charge me 350 and ends up doing it for 100. Doesn't sound too trustworthy to me.
And not only are materials not that expensive, if its a common color like flat black, that means he can throw it in with another order.