I would venture a guess of $325~$400 in the condition you describe. The Colt Police Positive is not so desirable as the later Police Positive Special.
I would ask $400 (or a "bargain" price of $395) and be willing to go down to the $325 level. If someone is willing to pay you top price, I don't feel it would be a "rip-off." The price is purely arbitrary. If you want a certain figure for a gun, and someone is willing to pay that figure, its not a "rip-off."