Apple Sliced is a site that keeps watch on App Store prices, and they've just released a year-end report on how much app prices fluctuated over the whole of 2012. Overall, says Apple Sliced, 35 percent of paid apps on the store ended up dropping their prices at some point last year, which is a sizable number for sure. A slightly smaller percentage of apps (around 32 percent) were cheaper at some point in 2012 than they are now, which means they dropped at some point and then raised. While these numbers represent a large proportion of apps on the store, they also aren't that surprising -- plenty of paid app developers do price tweaking and put on sales from time to time, usually just to raise interest (and thus downloads) in their apps.
Apple Sliced also allows users to set price alerts for apps, and it says the "most requested" apps for a sale are WhatsApp Messenger (which is currently free?), Smart Office 2, Plants vs. Zombies HD and Infinity Blade 2. That makes sense -- with the exception of WhatsApp (and Plants vs. Zombies, currently on sale for US$0.99), these are all very popular apps with comparatively premium prices.
You can see the same reasoning in the most popular app sales, which all feature premium apps that dropped down to the low price of free. Sales are a powerful tool for paid app devs on the App Store, and that's not likely to change as we move forward into 2013.