To be completely honest, my information is based on anecdotal reports from colleagues primarily located in Illinois. Across three universities in the state, every grad student I know in the biological sciences was paid between
$20,000 and $28,000 for the stipend. They then received full tuition waivers which completely covers the cost of tuition, which is between
$12,000 and $16,000/yr for PhD students.
Insurance premiums in the US for a healthy ~22 year old will be around $3,000/yr, give or take... but this is also included/covered by the university. That also is the bottom-barrel of coverage more or less, whereas my insurance from my university was actually pretty dang good.
That doesn't include fringe benefits (gym memberships, public transit, dental insurance, vision insurance). The low end of the above numbers is $37,000/yr and the high-end is $47,000/yr, again, without including what is probably a couple thousand more in the fringe benefits. I'd even say that the numbers I'm providing are on the low scale for the most part as I'm mostly looking at central Illinois, which is a relatively inexpensive part of the country.
I'm also not claiming that any student has a take-home pay of $60,000. Yes, taxes need to be taken out of the stipend; maybe it's just a US thing, but generally if someone asks your salary and they say "$80,000" that's how much they make pre-tax. Very rarely, at least personally, do I see people providing take-home pay numbers when disclosing how much they're paid.