like anything collectible, it's worth whatever someone else is willing to pay for it at the time you put it up for sale. Just because "the last one on ebay went for $XXX" doesn't mean yours will. Collector markets are unstable/fluid. For example, sell it around tax refund time, and it's probably going to net you considerably more than if it drops for sale after the holiday season.
It is a rare, really cool bike, and I'm sure you will have no problem selling it, just don't think this thing is your retirement plan. Cool that running into this at a rummage sale re-sparked your interest into old BMX, I don't doubt your PM box might get bombarded with offers of trade for really cool mongooses, and if that is something that might put a smile on your face, go for it. I'm sure somebody out there would dig the shit out of cleaning that bad boy up and adding it to their collection. Good luck, and don't forget to hit us back and let us know what ends up going down.