Steam, the folks who distribute Counter-Strike, launched a Dynamic Weapon Pricing market for the game. They explain it best:
The cost of weapons and equipment that you purchase in Counter-Strike: Source are now based on an algorithm that calculates the global market demand for various weapons. As more people purchase a certain weapon, the price will rise and other weapons will become less expensive.
Starting October 11th, the prices you pay for weapons in Counter-Strike: Source will be updated every Monday based on the volume of purchases over the previous week.
What's next — dynamically priced "Mage-Eye" Blunderbusses in World of Warcraft?
Dynamic Weapon Pricing [Steam]