This is totally self serving (http://news.ycombinator.com/item?id=3126173). If you're on the free plan, you should be able to max out your referrals at somewhere between 8-10GB of free space for you (they may have lowered it). I got 30 referrals for less than $10....give it a shot, I've had friends try this recently and they didn't end up spending more than $15.