Difficult to put an estimate on here in UK. A mint / fully working 6 pack should fetch a good price, due to rarity and 'old skool' collectabilty if nothing else. I've seen them change hands for $100 plus stateside
Always wondered if these were made in pressed steel, alloy sheet or a combination of both? If all steel it must add a lot of weight to a marker - even empty of 12g. I shared a bunker with a guy shooting one of these, while playing a woodsball day back in the early 90's. The novelty of getting pelted by his spent 12g wore off pretty fast, and needless to say I avoided sharing cover with him for the rest of the day