Dynamic Weapon Pricing is a new system that will change the way players of CS make purchases at the beginning of each round. DWP will monitor weapon purchases and then change weapon prices to reflect popularity. This means if everyone is buying the CV-47 its price will increase when the next update is made. In contrast, a weapon which isn't getting much attention, such as the MAC-10, will be reduced in price.
Updates to the weapon pricing will take place every Monday, and you can monitor prices using the new Counter-Strike Market website that Valve has set up. Here you can monitor weapon popularity and projected weapon prices, plus look at individual spending and plot graphs showing weapon prices over recent weeks.
The update should be patched into your version of CS: Source the next time you launch Steam.