That's interesting, they always scan my Costco card at the pharmacy. Wonder if mine is an exception.
In some states, I think there's a member price and a non-member price. Both of which are often lower than other places. But YMMV, state laws come into play so you have to check your particular store. Around here (northern VA) Costco has a rep as the cheapest. There are been several times I've needed people drugs for the dogs and they have always been lowest cost.