Used mag chucks are all over the place. The good brands are quite expensive when new; Walker, Suburban Tool, Magnetool, etc... As a result: folks often list them for crazy money regardless of condition. I don't see many that are in really good shape that warrant big asking prices on eBay.
I probably would be in the $150-200 range for what you've got (assuming still has good hold; hard to tell from pics how much meat is left on the top). I wouldn't be surprised to see someone easily asking $400-500 for that on ebay. That's just my SWAG though; there are different models and I'm not versed on everything that's out there. You could have a super-rare, high-end chuck for all I know? I'm cool with others chiming in on prices; I won't be offended.
For reference: I missed out on picking up an exceptional 6x18 at an auction recently. I was focused on work and didn't realize the end time for the lot. It went for $200. But that's auction prices.