in good condition anywhere b/w 4-450. just make sure the owner has maintained it properly. oil, filter, brakes etc etc. take it for a drive. the ride should be soundless and the a/c cold enough to make your earlobes hurt.
i own a 2005 model. it still drives like new and i love it.
i dont believe it ruining a car like chevy with CNG. personally, i would avoid buying a cng fitted one.