I'm pretty sure Dropbox paid a lot more than $7.95 for "dropbox.com" It's actually somewhat difficult to find an unregistered, pronounceable domain that's under 6 or 7 characters.
Dropbox had an already successful product operating on getdropbox.com. When you have some revenue numbers to play with, it makes sense to invest serious capital.
Is it actually important to have a short domain name? I either seldom visit a site, in which case I visit it through google, or I often visit the site, in which case it's in my url history completion list.
