Further from patio11's point - I'm very surprised Dropbox allow this, and wouldn't be surprised at all if they don't. I do a lot of affiliate stuff and 9/10 it is forbidden to used PPC (Adwords) to bid for the brand name, or any variation of it. Normally the advertiser will provide affiliates with a list of stop words that cannot be bid on to prevent this stuff occurring.

Still if there cool with it and you plan on keeping Dropbox for a few years this could prove to be a very cost effective way of getting more space :). Just be warned, if it's against the t's and c's then I would expect people will have their free space revoked.