How can you not be sure of the cost? It's on the invoice!
I presume you got a deal (or think you did) so don't want to say how much it cost, lest you creTe an imaginary problem for the gunsmith.
If I called them and said:
"Yeah, I want you to do exactly to my gun that you just did to his for exactly the same cost."
What will they say?
1) "Sorry, we gave him a once in a lifetime deal and we lost money on it and we will never offer those modifications for that price ever, ever again. Ever."
2) "Absolutely!"
Yeah, I think it's #2.
But if it's #1 and I still want the work done, I'll ask "Oh, ok, then how much is it?"
Keeping the cost a secret, unless sworn to do so, always puzzled me.....unless the buyer knew he paid too much and was embarrassed about it.