In USA, if you are lucky (good) enough to get full funding, it'll cover your living costs if you are single and you probably earn more than the national median. But that's an order of magnitude off from what you can potentially earn. As someone who is thinking of going for a PhD you probably qualify for high earning jobs. This totally depends on your degree. Say in Computer Science, where I come from, doing a PhD makes no financial sense in 99.999999% of the cases. Take this from someone who went through a long PhD program.