What would you pay for a 3D solid??

[astroracer]

If he want some CAD files and he is going to make a profit then you either charge up front or come up with a plan that you get a percentage of each part sold.

I wouldn't go that route... get a price that BOTH of you feel is fair right up front. The problem with percentage deals is he will "forget" about it. Believe me, it will happen, I don't care how straight up he is. It will also leave you wondering about it too, so just get paid up front and forget it. The other issue is, this is custom work, how many will he actually sell?
You should probably set a minimum cost up front. I know I can turn stuff around in 3D in a matter of minutes if the parts are fairly simple. Set a minimum (like 1 1/2 hrs) and then a base cost per hour after that. If the parts are simple I can see you making money on the minimum and just forgetting the "per hour" rate.
Just throwing some ideas out there...
Mark