If costs are the reason for lower fertility why are fertility rates highest in poor countries where such things (food, child care, health care) are even less affordable?
I'm guessing a little, but expectations might be different. The more affluent you are, the more you might care about losing affluence. The less affluent you are, the less you have to lose.
Child care can be social as well as remunerated. Grandparents and aunties don't cost you cash. They are social wealth.
Is food really more expensive for people in poor countries? You might not eat a European or North American diet, but aren't staples generally affordable in the absense of political and/or economic crises?
