We're trying to estimate this in all sorts of indirect ways, when the real questions, which I am sure we should be able to find answers for, are:
1) How many workers are there total?
2) How many workers are there in high-cost metro areas?
What we can't easily do is derive the answer for #2 from the total population of said metro areas; it's heavily dependent on the age structure of said metro areas and the specific workforce participation rates for those areas by age.
> Therefore, as you say, the "prime age" labor force participation rate is 65%
On average across the US, plausible.
> Applying this ratio to my original numbers, 45.5 million people are laborers in the Northeast
Yes, but even in the northeast not everything is in an expensive metro area, fwiw.
> We cannot possibly say that an area like Amarillo TX, with just 100,000 laborers, is indicative of some nationally sustainable trend
On its own, sure. But there are a lot more areas like that (by count; how the population totals look is not obvious to me) than there are big expensive metro areas.
That is, you might be right in your original claim that "half the country" are in the high-cost situation, but your data is not showing that very conclusively.
That said, I would be shocked if less than 1/4 of the country is in that situation. And yes, we should be aware that these tens of millions of people exist and of their situation.