Around here there are several shops so the pricing is very good, to get just the lock cerakoted I would expect to pay no more than $10, you might have to wait a week or two for the place to have a larger order of the same color black (cause they're probabily not gonna set it all up for your tiny little piece alone) but if you just tell them your welling to wait and have your one piece stuck in with a large batch my local show would do that for just a few dollars if I go when they call.
I like the idea myself, I have my second mini-grip on the way, when it gets here I think I'll get its lock done, but not black, I'm thinking red or even blue.