I don't see how that can be if the core costs - rent, medical care, education have all skyrocketed.
Diapers going down in price by 15% doesn't really help if at the same the price of college goes from $15k to $150k.
Wages are shit in a lottt of those places. Rents are reasonable as a % of SF programmer income I suppose, but the picture is a little different if it's mostly walmart jobs or nothing available.
The areas with high unemployment and high social problems are perceived as undesirable places to live in for a reason. Of course I don't need to live in area with low alcoholism rates, low unemployment and low amount of social problems. But I want to live there for good reasons, namely better chance for me and my kids to not become unemployed and hopeless.
What you don't have is a paradise where everything is cheap, there is a lot of employment and other opportunities and people somehow refuse to go to in the long term. You have cheap places where there is basically single major employer or low earning industry.
I am not saying that people always have it perfectly right, but in the long term and on average, the mass moves where it makes sense to move to. Those places then become more expensive.
Housing is a good example. Yes, housing prices in coastal cities have spiraled out of the reach of middle class people. Yet, there are thousands, even tens of thousands, of affordable cities all throughout the South and Midwest. The "problem" with those areas is that they are perceived as undesirable places to live. Again - a want, not a need.
The issue of inequality is a separate one and not the direct point that I am making. I'm simply saying that throughout all of human history, people were far poorer and yet still had children. Even today, the majority of the world's population is poorer than the poorest in the west. Clearly the main reason Western people aren't having kids today is not because they legitimately can't afford it - there are other cultural factors at play.