The Exams Sharing Portal uses the concept of "credits". You will need to buy 1 credit in order to enable viewing of 1 exam.
Once you'll have associated a single credit with a specific exam, that exam will be viewable both by the sending user and by the recipient user for an unlimited number of times, at least as long as all the exam's files will remain available on the sending user's Dropbox. You can even purchase several credits at once, and then associate each of those credits with an exam at a later stage.
Pricing is very simple and convenient: