anywhere from free-500$ (500 is as high as i would go for one in perfect shape).
to get it free you need extreme patience.
you could get lucky and get a cab for 75-150$ that's been converted and needs stripping but that's a gamble (but many times more fun and gratifying), you'll then need to source a control panel most likely and a marquee. the board alone will run you 40-90$ tops working with sound and correct cables.