For standard soda (Coke or Pepsi depending on location) Costco is rarely the cheapest deal at any given time but they are consistently fair. Supermarkets will have regular prices cases at nearly double Costco unit costs, but on sale they are moderately cheaper.

I’ve long since stopped worrying about saving a couple bucks per case while timing deals and simply buy from Costco instead. The couple bucks a month it runs me is worth not worrying about the mental overhead.