There are plenty of arguments for why a PhD is economically unwise for all but a couple of subjects, so to pay to do one seems crazy to me. Especially in the humanities where job security is far lower, or at least far less lucrative, than something like engineering.

What happens if after you PhD you realise you don't want to continue with academia - which happens to a lot of PhD students and there is no way you can be certain you won't be one of them - and you're saddled with an advanced degree that isn't much use and a load more extra debt?

Unless you are very wealthy and have money to burn - which you say you aren't - then I definitely wouldn't self fund a PhD, any PhD. Sorry.