Starting on October 11th, the prices of weapons and equipment in Counter-Strike: Source will be updated each week based on the global market demand for each item. As more people purchase a certain weapon, the price for that weapon will rise and other weapons will become less expensive.
Depending on how often a weapon is purchased, it’s price will go up or down. They do not say if this amount can be capped though. This could help spice up the game in certain regards, and force people to use different weapons instead of the same ole M4A1 or AK. There is a lot of interesting debate going on over on the official forums you may want to check out also.