A "Teaching Assistant" (AKA a grad student is TAing) is paid $45,138.00 per year, and a grad student who is currently paid out of grants earns between $42,729.00 for "Graduate Student Researcher - Step I" and $83,727.00 for "Graduate Student Researcher - Step X". I was slightly shocked, but this is the official salary scale. Granted, very few grad students are likely compensated at the upper end of that range, but still.
UC Berkeley [2] and UC San Diego [3] appear to be quite similar, so this is would seem to be somewhat representative of the UC system overall.
My own institution would seem to be actually lagging behind somewhat compared to this.
In any case, graduate student compensation has increased significantly in absolute terms over the past decade or two.
[1] https://grad.ucdavis.edu/resources/student-employment/salary...
[2] https://www.ucop.edu/academic-personnel-programs/_files/1819...
[3] https://grad.ucsd.edu/financial/employment/student-pay-rates...
I remember being a Graduate student in math at UC Berkeley in the late 90s, and getting just over $13K/year - it sucked (and grad students were striking back then)! So I also took out many student loans, which worked out for me because my salary once I got a real job was enough to pay them back very quickly (as I was used to living on so little money). I was a grad student at NAU in Flagstaff before that and got under $9K/academic year...
as a more typical example, the University of Illinois offers somehwere around $22k to CS grad students (and it's one of the strongest CS programs in the world), and $20k to math.
https://cs.illinois.edu/about/awards/graduate-fellowships-aw...
https://math.illinois.edu/admissions/graduate-admissions/fin...
I discourage everyone from doing a PhD, but it sounds like you've made your mind up.
Be very careful with your choice of advisor and department, and if it isn't working, be ready to walk away--sounds like you have a career to go back to.
But I'm mainly responding with this because a lot of "perks" of schooling like this are structured assuming you followed a traditional path -- namely, you're 18-24, single, no dependents, not much in the way of actual, adult responsibilities. The more you deviate from that the more inaccessible grad school and school in general become.