Basically , you should be about the same or 10% below what it would cost the customer to have the stickers screened and die cut.
Be aware that cmyk + a spot metallic can cost up to GBP80 per sq meter , work on that and x the cost of materials by 5
Your yield of 68mm rounds is 200 per sq meter – so in the worst case , the sticker would cost 40 pence , sell at 2 quid.
Obviously you have to take things on a case by case basis , if the guy wanted 5 of them , I would quadruple the price and if he wanted 10 000 , I would seriously sharpen the pencil.