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.

Dynamic Weapon Pricing [Steam]

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