This is an interesting idea, however, I doubt I would take a chance to implement such a plan or suggest it to a client. I am new to the business but from what I have read, it's hard enough to get clients to pay for a completed website, much less worry about whether or not they can be trusted (or to be more fair, "can be relied upon") to send me commission on every sale. Having a signed contract only ensures that I can take legal action if I can prove I am being cheated. Don't get me wrong, I like the idea of offering a client assurance that my work will grow their business. ....maybe I am too cynical, paranoid, or risk-averse in matters of business.