It is a very unique situation, indeed. I gave the option for % returned on investment or % of profit. They understand that the project may not make money and are willing to take that risk.
I suppose I could rephrase the question to ask "Based on the commonly used 'interest rate' payment structure, what percentage of your profit do you end up paying private investors?" I understand that nobody really calculates it like this, but its worth a shot.
If they are participating in the profits and not charging interest, they will be more like a limited partner than a lender. If you set it up as a partnership, normally, the limited partners (those who bring the money) would receive their money back and a preferred return on investment of 8-10%. After that, remaining proceeds would be split between the general partner (you) and the limited partners. This could be anywhere from 50/50 to 80/20 (80% to the LP's).