the cheapest way to pay

Z's dictum:

The cheapest way to get anything is to pay money for it.

Take childcare. Consider the effort and work involved in getting a group of friends to set up a babysitting club and keep it going. Consider the amount of careful, patient, futile effort it takes to try to convince your mother, when she babysits, to feed the kids what you want to feed them, to get them to go to bed on the schedule you set, etc. Or, you can pay a babysitter, and she'll show up when you agree and feed your kids what you say and get them to bed when you say, because it's her job.

The way this ties into the Samuel Vimes "Boots" Theory of socioeconomic unfairness is of course the kicker.