Valve has updated Counter-Strike: Source with a beta version of the "Dynamic Weapon Pricing" system it outlined recently.
DWP will see the cost of weapons and equipment that you purchase at the beginning of rounds changing depending on the behaviour of the global Counter-Strike community.
In other words, if everybody uses the M4A1 Carbine (Colt) one week, its price will rise and other weapons like the rabadged AK-47 will go down in price.
The prices will be updated based on the DWP algorithm every Monday based on the volume of purchases for the previous week.
Meanwhile, dedicated CS Source players can track the rises and falls in weapon costs through Valve's 'Counter-Strike Weapons Market' website.
As well as projecting prices based on market activity, the website also allows you to view individual spending and purchase totals on each weapon, and even a graph plotting value through recent weeks, allowing you to track weapon price and relative purchase volume in more detail. Loons.
Your version of Counter-Strike: Source should be updated whenever you next launch Steam.