I assume you have PP joints around you. Look at what the BBQ places charge by the lb, and knock of 30%. Tell him that in advance, I think it would seem like a fair amount.
Or, whatever you pay per pound raw, gross up by 30% (to account for cooked weight being lower). I wouldn't charge for the coal, if it's one butt and you're doing other stuff.
However, Gary's idea is pretty good. It could avoid uncomfortable situations in the future if he keeps asking. But I'd do it the first time and say something like "if you really like this stuff, I know where you can get a good deal on your own WSM and I'll show you how to use it, it's so fun, great hobby, stimulates world peace, blah blah"