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.