Given the work they've put into having clients for most things, and broad 3rd party integration, they probably have some room to overcharge for small amounts of space (sure, you could just go shove S3 buckets yourself; but we've already done the integration work for you...); but they seem strangely overconfident about their ability to set prices for larger blocks of storage that just emphasize the discrepancy in per GB price.
That may actually be part of how Google and Amazon and Apple are hitting them hard now: DropBox offers a reasonably compelling deal if you don't need too much space. Sure, it costs too much per GB; but it's all nicely wrapped up and integrated with the programs you use, and life is just too short to go tinkering or comparison shopping to save a small amount. However, Google, Amazon, and Apple will all give you a modest amount of storage for nothing, as part of their respective plans to sell stuff, achieve platform dominance, or whatever. That obviously cuts the knees out from under DropBox's formerly cushy margins on small accounts; and, as you say, their price/GB has always been sufficiently high that they really start to look bad if you need nontrivial amounts of storage(which nobody will give you for free; but which players like Amazon will sell impressively close to cost).