Given a "Could file sharing" service, is it feasible to provide non-secure (http) service for free, whereas if you need to store and transfer important data, you pay for secure service (https) a nominal ($5) monthly fee.
Would this be accepted by the users?
If you answer is "NO", please read my explanation:
A community-like initial response would be - no, you can't monetize of security, when it come to file sharing. But think about it - what are the chances that your traffic is getting sniffed on the way to ISP? Low. From ISP to the destination server data center - almost none. So when you are sending someone a photos from a party, you hardly need SSL. But if a designer is sending sketches of a web site, SSL is required when there are competitors, etc.