All pampers giga packs are on offer in Costco (got mine in Watford) it's not advertised in the booklet so was surprised to see roughly £8 off the ticket price.
They only had 'baby dry' that I could see and they had most, if not all, sizes.
Depending which size you buy depends on how many you get - I got size 4 with 120 nappies per pack working out to 8p per nappy.
In the interest of fairness - and for those not near a Costco - I think they might be the same price in Asda during their baby event and if you subscribe and save with Amazon they work out to be 8p too.
This is the cheapest they are!