Bathsheba, Asda's knicker packs---4 for £4.50 are okay.
I don't do a big shop every week, only 3 to 4 weeks so not that many packs in a year, especially if you change them a couple of times in the day, particularly if it's hot.
Because I boil them they probably wouldn't last as long as if you half-washed them in a non-boil wash, but mine are lasting alright anyway.
I've always had a thing about being prepared should I have the misfortune of going into hospital too. There are always two or three packs at the ready to throw into a bag.