As far as what I'd pay for it? :: shrug :: Probably not a good candidate to ask that. Where I live and play there is very little pump action, and the field being set up the way it is you'd be hard pressed to survive very long using a marker like that. So I wouldn't pay a whole lot as it'd just be a wall piece anyways.
But just for a heads up, MCB's blue book has that model listed as $75 for fair condition, $125 for good condition, and $175 for mint. Not knowing how well the gun shoots, what kind of valve, etc., I'd say you're definitely in the good condition range, plus a little more for the uniqueness of the vertical feed.