I think there would need to be some structure around children, what an incentive to make babies! $8k per baby per year, the Duggar family would be making $168k per year! I'd rather it be something as an adult. So the number of people 18 years or older is 242,470,820 [0]. Double the $8k to meet that poverty line as a couple. So at $16k per adult you come out to just under $3.9 trillion a year.
Edit: from the poverty data [1] you could say that only ~45 million people get the full $8k, the rest starts to taper off at the 50 cents to a dollar pace. That means this would cost considerably less. $360 billion is the minimum. The cost out would be $3.9 trillion, but those extra $8k checks would be collected at tax time. A one job two adult household, the job holder would have to pay back all the $8k, but the partner wouldn't if they didn't make any money.
Maybe. I think people mostly see children as a responsibility though, so I wonder if the problem would live up to the concern.
I wonder if you could try to measure the differences in birth rates for people receiving the EIC, comparing across numbers of children (at some income levels the first child provides ~2x the benefit as the third).
edit: Searching "EITC birth rates" shows that people have looked at it.
Edit: from the poverty data [1] you could say that only ~45 million people get the full $8k, the rest starts to taper off at the 50 cents to a dollar pace. That means this would cost considerably less. $360 billion is the minimum. The cost out would be $3.9 trillion, but those extra $8k checks would be collected at tax time. A one job two adult household, the job holder would have to pay back all the $8k, but the partner wouldn't if they didn't make any money.
[0] http://quickfacts.census.gov/qfd/states/00000.html
[1] https://www.census.gov/hhes/www/poverty/about/overview/