Well think of it as a customer acquisition cost and how many of the various services you would offer would you estimate you would need to sell/provide to cover that cost.I don't know how to predict a ROI on something like this, because I don't know how much stuff or work I will sell.
Interesting thought on the FFL part. What would be the concern of being a business doing work on firearm accessories/parts, but not the actual firearm itself? I don't know if I can get my FFL where I'm at, haven't pursued that yet. There's a whole process with the city zoning dept. I have talked to them about it though.
That would be a good starting point.