According to the 29th edition of the Blue Book, a 100% Official Police (Post War) in .38 Spl is worth $750.
The box will add a few bucks.... usually $20 to $50 depending on condition. Colt revolvers of that vintage were also supplied with a serial numbered trarget shot at the Colt factory and a blued-steel key ring screwdriver with the Colt logo on it.
Private sales generally go for about 80% of book, and a dealer will usually offer about 67% of book if it's something he figures he can turn around. If it's a dog, a dealer will offer 50% of book or even less.