Basically it's industrial ethanol with flavourings. It's not like cognac or something where you potentially start with a good wine and distil it. It is literally just a flavoured, diluted ethanol.
So the production cost could vary perhaps £1 or £2 at most between the cheapest Asda Smart Price and the super-duper-deluxe.
And blind taste tests repeatedly pick the cheapo ones.
metro.co.uk/2017/07/27/aldis-10-gin-declared-one-of-best-in-world-by-experts-6811653/
Just marketing.