Actually, you are using the wrong metric. Just because it's been fun, nearly free and interesting getting your Ph.D still does not address ROI. ROI is about how well your FUTURE income can offset the cost of getting the Ph.D, AND what type of lifestyle your future income will provide you, so no, what you wrote really hasn't proven that getting such a degreee is a good financial move.
I'm not saying it was wrong to get your advanced math degree....rather, I ask if you have researched the income and opportunities available to you so that you can have a great life which can only be supported by a great income.
ARMS: While I agree with you in principle I think your reasoning fails in practice. A PhD will almost never produce the optimum ROI in any field. If a person solely considered ROI from a financial perspective then we would have very few PhDs. While that might be desirable, and I'm not disputing that, I think we all benefit from research and education in ways that are rather difficult to quantify. Take quality of life - having achieved a PhD and being an intellectual makes you a member of a community of others like you and gives you a degree of respect in some corners that is impossible to get without passing that mark. How valuable is that? Not very in some ways and priceless in others. It is not just future income that can enhance one's lifestyle. It is also the sheer joy of knowledge, association with other scholars, and the understanding of the world that you develop.
Is it necessary to have a PhD to enjy those things? Of course not. But having one gets you in the door. I know lots of idiots with PhDs. In my current job in academia you can't swing a bat without knocking out a few Phds. In my former job, because of the people I associated with, it was extremely helpful and gave me great respect. 'm not just talking about ego here, the respect turned into authority that I was able to convert into business for my company.
In math, physics, etc, a PhD is like the entry ticket. It is effectively required to play in the game. It is the terminal degree. In my field, engineering, a BS is the terminal degree (though I think it is becoming the MS) and having a PhD does not usually earn you much more money. When you consider the opportunity cost of giving p a few years of advancement and experience it is even worse. So does that mean anyone who gets a PhD in engineering is stupid and made a poor decision?