How much is a webley mark 1 air pistol worth?

The Webley Mark 1 was made over a long period of time. 1924-1964. there were 2 variations and 6 different series. A lot of information is need to give you an estimate of value. it could be as much as $370 to as little as nothing. You didn't state the condition of the pistol, that and the series makes it possible to give an estimate. I suggest that you look up a book called "Blue book of airguns 7th edition" and locate your pistol to identify it. This should give you the information that you need.