These socks are currently ranging between £4.99 and £5.99 a set on Amazon Lightning deals (expires at 11.30am).
However, there seems to be some promotional offer which they haven't removed yet that adds further discounts on and if you add different socks to your basket, the discounts seem to add up making them pretty cheap.
Not sure if this is a bug or not so I suggest you hurry before they realise!
You can probably add more lightning deal socks to the basket than I added to get even more discounts but you can't have multiples of the same design. The discounts show up when you go to checkout.
My lightning deal basket should have been £22.96 for the 18 pairs but came to £13.35 when I went to checkout which is less than 75p a pair!
I believe this is only for Prime members.
EDIT. Thanks to iblackwe for confirming that it's not only for Prime members! See replies to see other deals people are getting.