Probably not very many. Typical postdoc pay is $40-60k/year depending on where the job is located ($40k in Urbana-Champaigne, $60k NYC or Chicago).
That's about 2-4x lower than what the postdoc could get in the outside world, but it's a far cry from minimum wage.
I say thee "Ha!", sir. The offers I was getting for post-docs were consistently on or below the low end of that range, even in San Francisco (which ain't cheap).
Probably one of the biggest nudges I had away from academia was the realization that as a post-doc, I'd have a lower standard of living than I did as a graduate student. It didn't seem like a smart trend to follow.
I guess applied math is the place to be if you want to stay in academia.