my car has a knock, and i was just thinking that it may be a knock sensor that needs a replacement, and i was wondering how much that would cost, and if it is something that i could do in a day.

The knock sensor detects knock it does not make it. The fact your car is knocking is very bad, there are alot of things that can cause this and you need at least a stethoscope to narrow it down. I suspect valve train, in particular a bent push rod?

Have you been running the engine hard at high rpm'*?