No, there is no general rule, you're in wonderland. You can't solve this riddle with an established formula. You have to use common sense and gut instinct and if you have the benefit of somebody else with experience in this area, ask them. Do you have an attorney with small business experience? An accountant? These are shark-infested waters, so don't go in there alone.
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.