If I remember right from the deal made with ICANN, the price is supposed to increase 5-10% each year for 5 years. I can't remember the specifics of the deal, but I remember that there was some talk on this a few years back. Maybe othello can shed some light on this as I believe his company is an accredited domain registrar with ICANN.
From memory, there was something a few years ago that ICANN granted Verisign the ability to govern their own pricing for .com and .net domains for the next "X" years, and shortly there after there was an announcement that they'd be increasing by up to 10% each year.
Still, much better than the $75/year and $35/year that I used to spend back in the 90's