Compare produce prices at a farmers market versus the store.
Market produce often carries a higher shelf price but a longer usable life, because it was picked days rather than weeks before sale. Since you shop roughly weekly, anything that will not last seven days at home is partly waste, and waste raises the effective cost of what you actually eat. Dividing the shelf price by the fraction that survives to be eaten puts both options on a comparable footing, and it frequently narrows or reverses a price gap that looks decisive at the till. Shelf life, not headline price, is usually the deciding variable.
Waste fraction
Waste = 1 - shelf life days / 7, floored at 0 and capped at 0.6
Effective cost
Effective per kg = shelf price / (1 - waste fraction)
Because you pay for what you buy but benefit only from what you eat. Produce that wilts on day four of a weekly shop delivers only part of its value, so its effective cost per eaten kilogram is higher than the shelf price. Longer-lasting produce can be dearer and still cheaper in practice.
Usually but not always. Growers selling directly typically pick within a day or two, while supermarket produce may have spent a week or more in distribution. But some market stalls resell wholesale stock, and supermarket cold chains are excellent for hardy items. Ask when it was picked.