He wrote "for about 20 bucks", so I suspect he spent around $20. And "maxed out my Dropbox referrals" seems to imply that he did it up to the maximum for Dropbox referrals, which is 8 GB.
You've got the first part right, but I also happen to work for a university so I associated my .edu email address to my Dropbox account. Doing that will double your referral bonus to 500MB per referral, and double your maximum free space from referrals to 16GB.
This is a good way to burn those $100 AdWords credits that Google likes to hand out like candy.