Master's degrees are quite expensive. I would suggest gunning hard for a RA position direct from undergrad. Barring that, I would seriously consider your return on investment from pursuing the path your suggest above.
I looked over your profile and you should be competitive for RA positions now. Apply extensively, to the NBER postings, Fed postings, etc. An RA position will also allow you to better assess if research is really something you want to do, which is the most important determinant for pursuing a PhD.