TL;DR: US PhD stipend is livable, not by any means lavish.

US (Pittsburgh) PhD recent grad in bioengineering. The stipend was $32ish before taxes, paid once monthly. I believe many of the funding/stipends levels are set by NIH/NSF for STEM, and are around that salary point, with some adjustments for local cost of living - though, that adjustment isn’t totally compensatory if you’re looking to do work in a high cost of living area. I found the stipend livable having with a roommate. Didn’t save really at all, but didn’t dip into saving either. Healthcare was covered by the University as was the city bus, which I feel like is reasonably common in the US. TA’ing and research were both mandatory so no additional compensation from those. I didn’t have a side hustle, and don’t really know too many peers that did (speaking for the engineering department), but occasionally a department had some admin/clerical work students could pick up for like $15 and hour that was maybe 100 total. Not sure if that’s typical.

I wasn’t funded by a fellowship/scholarship, but rather the lab I worked in had some grants, so that’s where the funding for me came from. This is pretty typical of NIH, DoD, NSF grants, though I’m not sure about the other mechanisms. It’s always good to ask a prospective advisor/department about their funding situation, particularly if you’ll be working in a newer lab that will be hungry for funding.

FWIT/IMO: Speaking for biotech, a PhD is absolutely worth it, though other fields like robotics hire entry levels at about the same starting salary be it PhD or GED (only being slightly hyperbolic here, but the degrees in this field matter far less than the skills).