Chrome Webleys are worth less than the black ones. All Webleys were originally black, but a large amount of them were sold as war surplus to Federal Laboratories and converted into gas guns for anti-riot use. Those converted guns were chromed and the stocks were stamped Federal Laboratories.
Mine was dechromed and converted back to it's original gun blue finish, but the stock is still stamped.
Prices vary a lot on the Webleys so I'm not sure if you could actually quantify the price difference.