Frank is correct about the second contract serial number range but there were many more thousands made after the war and will typically not have the Astra logo on the slide but marked 600/43.
These guns are worth less than the German contract and can be had with a holster and spare mag for about $250.00. So if the serial number is above 38,500 it is worth less, in my opinion.
You can also find some of the German contract guns with a matched mag or two. In my opinion this will increase the value by 20%.
Many of these guns have been refinished. The trigger and safety lever should be in-the-white as in this contract pistol.
http://www.p38guns.com/Astra600.htm
Mark