I think the most important test is "do you care deeply about or are fascinated by the subject in question". If the answer is yes, then a PhD is not a bad idea (conditional on ok job prospects, inside or outside academia). A PhD will almost never be a better decision than a Masters degree from a financial perspective. But it can result in a different and perhaps more interesting career even in industry.

If you are considering economics, one thing you have to understand is that econometrics is very oriented towards economics, and if you are more interested in statistics per se, you would probably be better off in a stats program. There is very little machine learning in econometrics even today.