I feel like $80k must be an outdated number. My wife and I pull in just under $100k together, and we can't even (comfortably) afford a home.
Between rent, student loan payments, health insurance, bills, and car loans, and food, I think we maybe have $500 left to throw around each month. If we were paying against a mortgage, that would basically be zero.
It's definitely outdated, or at least location dependent. The average rent in Manhattan in 2/16 was $3,850. Landlords won't rent to you unless you earn 40x the monthly rent which is $154k. In Brooklyn, the average is $2,716 which ends up being $108,640. (http://www.mns.com/brooklyn_rental_market_report)
Most people don't live in Manhattan or Brooklyn. If you live in a high COL area, adjust accordingly. I suggested using an approximate 2.3x multiplier elsewhere.
The most financially effective way to be a programmer is to work remotely for someone who pays SF or NYC salaries and live in a moderate COL area.
I've always seen the rental income requirement you stated phrased as "monthly income must be at least 3x monthly rent", in case the 40x thing threw anyone else off (40/12=3.33).
Mortgages are usually cheaper than rents on comparable units. The issue is that they require a big upfront chunk of cash to get into.
And, as a sibling comment stated, it depends on location. I think $80k is right for most areas of the country. If you live in NYC, SF, LA, or another place with really high cost of living, adjust accordingly (which is probably like 2.3x).
There are whole communities of people dedicated to answering this question [1] and most of them seem to think that they'd be content with $40k/year in passive income.
Between rent, student loan payments, health insurance, bills, and car loans, and food, I think we maybe have $500 left to throw around each month. If we were paying against a mortgage, that would basically be zero.