I emailed him an hour after the original auction was created, trying to buy it with cash, hoping I noticed it before everyone else. I would have gone up to $500 or so. Of course, I was only one of hundreds, so he killed it and relisted it.
It wouldn't surprise me a bit if it went for $1800, but in my opinion, he'd do better leaving it alone or starting the relist at $1, so a bidding war could take place. It is possible no one will bid on it, and again he will just get a bunch of cash offers in email.
Anything less than $500 would be a steal for that game. $1000 or more is about right. The upper limit just depends on how many people are dying to get this game right now.
Doesn't matter if it's a prototype or not. It is an extremely rare example of a very desirable game. Nice regular Defenders go for $600-800, so this one is worth AT LEAST that amount. The problem with the game is minor and won't affect the cost whatsoever to a collector.
Wade