I remember a poor condition example of the free gift Big Bang Fun Gun being sold (Sep 2004) as part of an auction lot so at least one exists! If you've got one, add whatever percentage you like to this value!
In 2009 two more examples of the free gift gun came to light and another in very high grade was auctioned in Mar 2010. It went for £193. Should a Fine copy of the comic and a gun come up for sale, you could expect £400 Plus value.