Theres 1 for my car up on ebay for $40,00 and i was wondering how much more HP do u roughly get, and i hear the gas milege inscrease aswell, if so roughly how much? Thanks!

maybe 4-5? i have no idea

Nothing that you'll really notice. Basically it's the first step into modding. It helps the engine breathe easier so it can fart better. It will increase gas mileage some. But with a Tuner you'll get the most out of increasing gas mileage.