Probably around $300.00, if it is as new condition with not a speck of dust on it. The Toppers, even dressed up, never commanded much in the secondary market. To be honest, if buying guns was not such a pita in NJ, I'd make you an good offer just because I shoot a lot of trap and have bought all kinds of interesting guns before NJ turned completely communist.
My suggestion is to sell it locally to a burgeoning trap shooter as their first dedicated trap gun. Every trap shooter wants a Browning BT-99 but can't afford them. That Topper Trap is BT-99ish and will make some kid smile. Even with an exposed hammer. Never seen one on a trap gun before.