While not general rules, here are some tips that might help: Call this a finder's fee and do a web search on finder's fees, and discussions of finders fees. It better not be very much. Most people who bring others together, resulting in a partnership, do it out of good will, networking, and assuming that a favor will be returned later. There isn't much economic value there. If you pay too much, the partnership fails because of it.
Tim Berry is the chairman of Eugene, Ore.-Palo Alto Software, which produces business-planning software,