Yeah, with EM's there isn't enough sales and the market is so small that they're hard to find and hard to price... I like the look of the game, but it would all depend on if YOU like the look and theme of the game! $500 for one an Aqua Gun that works and very clean would be about max in my world, if you could get into it for $300 I think thats more than fair... From my observations $300 easy for a working mechanical game, then they go up drastically if its a sought after or popular one.
How much is he asking for it? The game looks pretty cool to me, and you could always keep it until a shooter with a theme you like more came along and sell the Aqua Gun.