its $18 because its just a fan, the ones you were looking at on newegg are fans with heatsinks. $18 is usually alot for a fan but that is a really good fan, a standard fan does about 30cfm that fan does 125cfm, it moves alot of air. but its also pertty loud.
I would get that fan drill a hole in the top of the case and mount it there. it Will pull all the hot air out of the case that is just rising to the top and going no were.