One thing I've always wondered is what is their actual cost structure. Based on my own usage I would assume the actual storage part is quite small compared to the data processing and bandwidth. Since the sync works so smoothly, it is convenient to keep all kinds of projects there. Thinking for example node.js projects, which take not so many megabytes, but may contain 10-100k small files (due to dependencies).
Have we really come to the point where its "couragous" to charge money for your product?
Example is Twilio. They were first and we implement them very heavily into our systems. Currently our bills are at around $400 per month. Phaxio is much cheaper, but we just got used to Twilio and unwilling to spend time money and resources into jumping to something else just for the sake of little savings.
They've always charged for higher tier services but they've also always had a free tier where they provide service without charging anything.
If I had to guess, Dropbox improves their margin by getting people to share files and folders (because that counts toward multiple users' quotas).