I have a costco card and it is definately worth it. Twice a year they do special deals on Johnson's baby wipes, Huggies and Fairy non Bio.
Got huggies for all my antenatal group last week, it worked out at £11.50 for 210 huggies super dry size 3. They had all sizes up to 5 that I noticed.
Johnsons baby wipes work out at 75p a pack when they are on offer.
And I got 11.4kg (120 washes) of Fairy Non-Bio for less than a tenner.
All of these prices are considerably cheaper than supermarket prices. Although you do have to be careful and know your prices as not everything is always cheaper!