I am limiting my answer to the scope of engineering PhD programs. In the USA, the stipend number varies greatly by geographical area, university prestige, private or government funding, and program of studies. I know of universities that pay annual PhD stipends from USD $10k up to USD $55k. These are normally dictated by the amount the university determines as "cost of attendance."

This amount generally caps the available financial aid (in the form of scholarships, fellowships, grants, etc.) that the university can provide to a student. Of course, there are external sources but that wasn't part of the OP question.

In my impression, the (roughly) top 50 engineering programs will pay a decent enough stipend for a single person to live comfortably (rent a private studio/apartment, eat out regularly, make car payment, etc.) without splurging.