Costs are also nonlinear. The most nonlinear are support costs. You may pay as much to support the user who pays $1 for your service as the user who pays $100. (Unless you keep a meter running and charge by the support-minute, which makes customers very unhappy.) And in practice it's even worse than that: Anecdotal evidence suggests that the customers who are obsessed with saving money cost more to support.
And let me emphasize that costs aren't even the key issue. Engineers tend to forget this, but the price of a product need have little to do with the cost of producing it. Products should be priced based on value to a customer, and that doesn't scale linearly either. The first few hundred MB of Dropbox are the most precious of all, partly because that's enough to sync my most critical and frequently-changing files, and partly because it includes the installation and setup overhead (and, probably, most of the support costs). Then the next few GB is somewhat less precious. By the time we reach a 49GB plan, the value-per-GB is significantly lower. I don't have 60GB of important stuff. Moreover, Dropbox is significantly less convenient when shlepping around huge amounts of data, because it's not magical: It takes time to sync all that stuff over my lame-O cable connection.
Personally, I hesitate at $10 a month when I am currently using less than the 2GB of free space available. If there were a $5/month plan that was even minutely better than the free offering, I would sign up without a second thought. I suspect I am far from the only one in that position.
I think those prices were the cheapest viable at the start but it feels like there are more competitive ones now (and that's assuming they're not building their own cheaper datacentres).