You always hear "it would cost more to stop and start my car than I'd save by going to two stores"...but how true is that? I can't seem to phrase the question in the right way to google it, nor can I find the answer on a credible website.

If I'm shopping at a grocery store, how much would I have to save on a couple of items to make it worth ALSO stopping at a drugstore for shampoo and toothpaste as opposed to buying them at a higher price at the grocery store? (Not counting time and effort, just actual car costs)

In other words, does it cost 50 cents to stop and start the car? A dollar? Two dollars? I'm assuming that the amount includes not just gas but wear and tear, etc.