Nadia Taha, a finance writer living in NYC, calculates the total minimum cost of her and her spouse raising a child: $1.8 million. In the defense of high amounts such as that one, KJ Dell'Antonia broaches the problem of our aging economy:

[E]very one of us, parent or not, will at some point depend on today’s children to provide our medical care, to pick up our trash, or to grow our food. An economic exchange of money, goods and services isn’t possible unless there is someone willing to make that exchange and capable of providing the service or goods you seek. "Capitalism," that masterful force, can’t function without fresh people on which to exert its incentives.

Nancy Folbre reframes the macroeconomic case for reproducing:  

Capitalism, that masterful force, would have to pay much more for workers if families weren’t willing to pony up most of the time, money and effort necessary to raise them, train them and educate them.

She concludes:

Our standard economic accounting system ignores the value of goods and services that lack a market price – including clean air and a stable climate, as well as the health and well-being of our human resources. Nature instructs us in the danger of ignoring the value of unpriced inputs into the output we label gross domestic product. Efforts to assign a dollar value to ecosystem services are analogous to efforts to assign a dollar value to the work parents do and better estimate the costs of children.

James Pethokoukis looks at how tax incentives for parents might address this:

[B]y raising and educating their children, parents have already contributed hugely (in the form of human capital) to social insurance systems. The cost of their contribution, in both direct expenses and forgone wages, is often measured in the millions. Requiring parents to also then contribute to payroll taxes is not only unfair, but imprudent for societies that are already consuming more human capital than they produce.

So one option is giving parents a break on payroll taxes. The more kids you have, the less you pay. Another option is a new, larger child credit that can be applied against income taxes and payroll taxes.