How much to compensate a private party for a silent investment?

[Darin Groom]

My wife and I have been flipping houses going on two years now.  We now have private parties wanting to invest in our business.  We are doing multiple properties where we are needing additional funds in order to flip one property while another is being sold.  What is a fair way to compensate an investor for their silent contribution to the investment? For example, if we purchase a property for $150K and need an additional $40K for rehab, what is a reasonable way compensate a private party for their portion of the investment?