[thebigjc]

There's two ways to do it:

1) Go simple - affiliate fees in ecommerce range from 4-8%. Pick a number you're comfortable with and use that.

2) Do the math - estimate their revenue per year/month/week, do a Net Present Value calculation, using a discount rate based on the fact you're getting paid over time, and the fact that you may never get paid. Divide your quote for option A by the NPV, and that's your % to ask for.