I haven't looked at Aperture or Lightroom, but iWork's price change isn't quite as dramatic or inscrutable as you think: the retail package was $80, and the 3 App store components are $20 each. Apple likely lowered the price as an incentive to push users into its App store, where it gets a cut from every piece of software sold. Dividing the components and selling them individually probably brought in quite a few new users such as myself who didn't want to pay $80 for the full iWork package since it's only Keynote I'm interested in.

Services are more deeply and negatively affected by price changes than software, though. A price change that seems insignificant to you can drive many users away. Look at the fiasco with Netflix last year; they changed their prices by about $6 and subsequently lost about a million subscribers within a few months. Some users see price increases as an indication of a service's instability in the marketplace (and therefore a cue to jump-ship). Most users have lower tolerance thresholds for price differences when comparing services than they do when comparing one-time purchases (like software).

Regardless, now would be a really bad time for Dropbox to increase their pricing. Many Mac & iPhone users are watching iCloud closely and realizing that it could make Dropbox redundant for them. A price increase, even one as small as a ~$30 a year change for their 50+ plan (corollary to your $3 a month change), could push enough users away from the service that Dropbox is better off sticking to its guns.