Cost to raise man: $200,000. Worth of man: $700,000. Crazy profit margins. Salary's low, though. Only $9000 per person per year. Today I'm neglecting the intangibles and the fact the parents tend not to capture much of the material value.
However, the more children you have, the better the salary is. Two children: $13,500. Three children: $16,200. Eight children: $21,600.
Zeroth, note this puts an upper limit on your country-wide average salary. It can't be any higher than some constant * the average profit/kid because there's nowhere else for it to come from. (Unless central bank &c.) There are no activities that produce value without human labour. Average salary is not exactly the same as profit/kid because the generations overlap in practice.
In a non-Malthusian economy, the bigger your families, the richer your country will be. (P.S. population decline is eugenic and in the long term most modern families need to die out. It's worth the temporary privation to get long-term genetic gains. It won't be enough, but nevertheless.)
If this calculation hasn't neglected anything important, we would expect that those with larger families are more satisfied than those with smaller families. The only child seems enviable because they get all of their parents' attention, but in practice that attention is negative or corrupt because they become a reminder of the parent's genetic failure, and thus become a outlet for their stress. Children clearly crave more attention than they need for some stupid reason; alternatively in a large family they can get plenty of attention from their siblings. Children have more free time so you would expect that in a large family, there is more attention available. As long as they're not in school, anyway.
Meanwhile, parents of large families note that it becomes easier the more you have. Apparently the tipping point is around 2. The first two start to help to take care of the third. You also start to get economies of scale. The difference between 0 kids and 1 kid is, to oversimplify, kid-proofing your house. For the second, the house is already kid-proofed, you just need a second bed. And so on.
I think folk have few kids mainly as a status symbol. "Aristocrats in the 1500s had small families, and Every Man a King, therefore I must have as small a family as possible." Likewise, peasants had to have tons of kids because so many died that in most parts of the world they didn't even name the kid for a year or two. They don't think about what they're doing or get feedback in any way, it's manic status-striving combined with paranoid conformity.
Because children are profitable, being able to afford fewer kids is a way to show off your above-average salary. Social status is relative, not absolute, so problem: at most half of parents can have above-average salaries... Not to mention the dysgenic effects of an inverse relationship between salary and family size.