I paid $175 for a NBA Jam Cabinet that had been converted to a Run and Gun 2. It was missing the game boards, but everything else was there and in good condition. The monitor is a front-mounted 25" monitor. I think I got a pretty good deal.
As for how much I would pay for an empty cabinet... that depends. What are your plans? If you want to restore the defender cabinet to its original form, then you might want to pay to find one in good cindition, with good side art. If you're planning on making a MAME cabinet, side art might not be important, so you just might want a cabinet that's structurally sound. You can usually find good cabinets for free, or at least next to nothing from a local collector, or a distributor. Check around. I was offered at least 10 different cabinets for next to nothing before I decided to get one with a monitor.
Daniel