Why are some xbox 360 games 60 dollars and others are less?

Well, my question is pretty straightforward. Some xbox 360 games are 60 dollars, such as halo 3 or gears of war, and others are 50 dollars, like call of duty 3.

var:http://x360stuff.ditttos.com/xbox-360-questions-and-answers/why-are-some-xbox-360-games-60-dollars-and-others-are-less