Ask HN: Typical referral rates

I am trying to enter my pre-launch startup in a b-school business plan competition. One of the things I am trying to do is project new user growth based on referral rates. Dropbox would be an "extreme" example of a wildly successful referral system. What does the median look like in the referral marketing world? Specifically, what percentage of users might be interesting in referrals and how many new users do they bring in on average?