Hmm, that's an interesting idea. I have a similar plan to go to law school when I'm older, for fun (obviously assuming I can get it for free/cheap).
I'm currently doing a PhD. One thing to think about -- do you have the intention of getting funding to do your PhD? Depending on your age, I wonder whether departments would be interested/willing to admit you. From their perspective, they want people who are interested in being in academia, which they might think doesn't align with someone who is already 40 or 50. Not saying this as discouragement, just food for thought. Also, there is a lot that is lonely & hard about doing a PhD ... you could do a lot of the independent pursuit of learning & knowledge on your own, without doing a PhD program.
Also, as far as funding goes ... stipends can be really generous. Mine is 31K/year (but at a top program, and my field is quiet lucrative, much more lucrative than humanities generally). On the other hand, I hung around a lot in the econ PhD forums when I was applying (I am in a similar field), and some of the schools have stipends that are more in line with 12K.