Dating Site Referral System

We are looking to implement a referral system into our dating site. This system must be able to assign unique ID's to registered members that can be used by them (probably as an url ie [login to view URL]) that can be sent to friends via email to sign up for our services. Once somebody clicks on that link or enters a members name on the sign-up page, it should be tallied somewhere that we can monitor (preferrably the backend.) This system must only track paid memberships. We plan on giving away prize money to whoever refers the most paying members, so it has to be accurate and foolproof. We use Paypal and CCBill to process our payments and this system must be able to differentiate between a free user and a paid Gold member, and only give credit for the latter. This system must also display the number of referrals on the member homepage so they can see their progress. Site is written in PHP, with a MySQL database. If it is going to take you 6 days to complete it, please bid accordingly. I don't care, but I would like to have a realistic timeline to completion. Preference will be given to those that can show similar work they have done.