Yes, Penny. You have hit the nail squarely upon the head there. Savings here and there and higher costs there and here...within the same store makes comparisons over the totality of the weekly shop a mine field.
What may help is a local shopping list or shoppers club. Whereby news of low prices could be shared locally or shopping trips could be shared. One person taking two lists to one store another with the same two lists going to another store. Then having a share-out?

Ignorance is bliss, bliss is happiness, I am happy...to draw your attention to the possible connectivity in the foregoing.