[chjohasbrouck]

One caveat to this kind of thinking is that money is fungible.

One can easily pay for a vacation or years of eating at nice restaurants every day all in cash, and also go into debt for an education.

Also, one individual taking out an auto loan and a loan from Sallie Mae isn't 1 good loan and 1 bad loan. You could've foregone the high-interest loan on the big depreciating asset and used the savings to fund the education instead of the loan from Sallie Mae. In that scenario both loans were probably a bad idea, regardless of the return the education nets you.

I think another key metric beyond interest rate and rate of return, is degree of necessity. That's harder to measure though.