I think this popular narrative of "kids are too expensive" is mostly a media fiction. It's more about individual comfort. Money can be a factor in comfort, but it's hardly the only one.
Poorer immigrants tend to have many children. Even today, poorer people in general have more kids than wealthy folks. Just look at the data - it contradicts the headlines.
As Peter Zeihan likes to say, kids were free labor for the farm and made life easier. In today's world, they're expensive furniture for your condo or apartment and they break things.
People are focused on their own lives and careers and aren't ready to take a back seat and turn attention to children. They're main characters. I think a lot of folks feel that having children turns you into an NPC.
Poor people aren't climbing the corporate ladder. They don't need to worry about keeping their calendars clear for meetings. They're not filling their Instagram with travel photos.
Middle income folks have access to social media and easy dopamine hits. They want to stretch being single or unmarried or not having kids out longer and enjoy it more.
It's society, not wealth and cost of living, that is causing the dip in children.