It is also a sunk cost. Dropbox has already paid for your hard disk space - they run their own data centers. Okay, there is an opportunity cost, but unless Drop

It is also a sunk cost. Dropbox has already paid for your hard disk space - they run their own data centers. Okay, there is an opportunity cost, but unless Dropbox is running really close to capacity in its data centers, it is quite a small one.