Gary, I doubt a small shop can afford it. Any business, that the insurance companies think that can easily lead to a liability suit, is high. The insurance on my machine shop was $1,000 a month for a $1 mil policy. I haven't checked on Gunsmithing alone in a while, so I'm not sure what it runs now.
In my opinion, you would need a gun store, along with gunsmithing, to generate the money to pay the premium. There may be cheaper policies, but the coverage will drop.
Edit: If they still have it, you can get it through the NRA, and I think there's a policy from a company in Philadelphia. My guess is $1000.00 a year, at least, for decent coverage. 20 years back, it ran around $500.