This is really helpful information, thank you!

$30/card out the door isn't bad, although I wonder what the power draw is like. I've noticed my SFP+ 10gbit cards and transceivers get quite warm (different models from what you linked).

I'm not sure about power. The only way running these old machines is economical for me is because I have solar... that 2013 haswell machine draws about the same power as a znver4 machine I built last year, but it takes several days to build a Yocto image the znver4 builds in two hours.