I plan to pay for the courses. I could also potentially get my employer to sponsor some of the courses relevant to my job, especially in statistics/econometrics.
And yes, it seems I was overestimating the coursework that I could do part-time. Firstly, as you point out, it may be logistically impossible to undertake many of them. The core courses at e.g. Stern's finance PhD program, NYU Stern | Error, are different from the MA ones which are typically offered in the evenings, despite having similar course names: http://econ.as.nyu.edu/object/economics.0812.ma.courseofferings. Secondly, full-time job + PhD level coursework may be too intensive.
So at best I could probably do two/three courses per term among the few core + elective courses offered in the evenings.