The reality here is that the cost to operate doesn't matter to Google. They can keep subsidizing Drive with revenue from Search Ads long after they have pushed Dropbox out of business, if they choose to do so.
This is why Dropbox (smartly) does not want to compete on price -- instead they want to compete on quality. There's no way to be cheaper than Google, Microsoft, Apple and Amazon in the long run, because they each have other businesses that are licenses to print money. If one or more of them decide to heavily subsidize their online storage product, they can outlast you.
However, it turns out that all of the money in the world can't magically make a great product. You still have to actually do the hard work.
Interesting how I'm getting downvoted for stating this. The language does come awfully close to explicitly stating just the thing:
"We collect information to provide better services to all of our users – from figuring out basic stuff like which language you speak, to more complex things like which ads you’ll find most useful"
even explained thus:
"For example, if you frequently visit websites and blogs about gardening, you may see ads related to gardening as you browse the web."
and they say that they base it on:
"Information we get from your use of our services."
Again, not explicitly named, but not forbidden either by their own privacy policy.