Dropbox is a poor example to generalize from, since in a very real sense they're a cloud provider. They make bank directly on server administration, so cheaper servers translate directly to the bottom line. They would have to be incredibly foolish not to in-house this at some point.

Most SaaS etc. businesses make money in a way that is indirectly related to their server costs. Bringing the cost of servers down increases margin ceteris paribus, but the Latin is doing a lot of heavy lifting here.

It's not at all clear to me that the median AWS customer can save money by going self-hosting. If I had to bet money on it, I'd bet against it.