Perhaps money alone is not a reliable factor, and there are certainly confounding variables, such as poor people having low access to healthcare including contraception and education about options and how to use it.

More important than money is economic security, the ability to expect a reasonable long-term access to a sound source of income.

Having to worry whether you'll be laid off next week and not be able to get new work, and have that worry be constant over a decade is a real discouragement to having children.

Having a stable situation in life is vastly underrated, and not easily measured by current net worth or income.