As has been recently announced, Valve have added a new way of buying weapons to CS Source. Dynamic Weapon Pricing sets the price of the weapons and equipment in the game based on the demand for each item. Prices are calculated weekly and may rise and fall depending on how many people have bought them the previous week.

This is a pretty clever feature imo, but it remains to be seen whether the CS Source community will embrace it. You can check out current and expected future prices for weapons at http://www.steampowered.com/stats/csmarket/