Good that you limit it to the basic package only. I've done the % off thing before and it's always painful. I'd rather give a bonus item that they perceive at a higher value. What I've always heard is be stingy with the discounts....it costs you more to give a % off or $ off than it does to add on a bonus product.
Honestly,, what im thinking about doing, 3 referrals and the print package is free. My total costs for prints is less than $35.00. All im out is the cost of the prints and post processing time. For 3 paying referrals, why not ?? I want them to buy albums and wall prints so why not give them an incentive to do just that. Im even considering 2 wall prints gets 50% off the print package, 3rd wall print gets the print package for free. Prints are cheap. Clients know they are cheap. Why not take all the BS we put into why we charge so much for them to better work. By the wall art or buy the album and get the prints for nothing is what im thinking.