To be fair, it does drop in price literally every month to about $5-$10, and thousands of people already have the game (because it's been out for a very long time). If you're stupid enough to buy it at full price then that's your own fault. And that's the thing, if you're new to Steam and you see CS:S and it's $20 you're gonna be like "well im not paying that much for a 7 year old game", so when the monthly price drop eventually comes around, you're then like "oh well that's a good price, I'll buy it", and loads of people do. So Valve isn't counting on anyone buying it for that price, they're counting on loads of people buying it when it drops in price.

It's a pretty clever tactic, and it's obviously working since yesterday it was one of the top sellers.