At those rates, Google's service for 100GB for a year would be $25. Dropbox's would be $240. I think I'd call a little under 10.5% the cost "dirt cheap", but I guess that's subjective.
In fairness, when you compare 2 things, you should use the same units, and they didn't. They should have extended Dropbox's plan to the year to show the difference.
to be fair the original pricing for app engine was for a google product that was in early beta. Google Docs has been out of beta for a long time, and their storage prices have decreased since they were first released. In addition they have removed file type limitations, and increased max file size to 10gb. It is be beyond unlikely that they would regress and increase prices.