I went from the MPhil in Development Studies straight to an Econ PhD, and I know a few other scattered cases, though it was a long time ago so I’m not sure if that path would work these days, and it was pretty unusual even then. It may be doable depending on your undergrad coursework and where you’re aiming—admissions committees care a lot more about having a math background than they care about having an econ background.
Question: are you looking to continue on to a PhD later, or go to the international organization / non-profit sector, or something else? For most work in international development you don’t need a PhD and the MPhil would work just fine to get you in the door.
I wouldn’t assume the MPP would significantly help you get into the MPhil in Economics. And if you’re set on some kind of second master’s after the MPP, but not after the MPhil in Development Studies, the opportunity cost of delaying your career by 2 years would be much higher than the difference in the course costs.
I would agree that econ has the potential to have more impact than a development studies degree, but neither program is an econ program. (Maybe that specific MPP has a lot of econ content, but MPP programs in general do not, and if this one does I would not know.)
If you are going to work for an international organization, either the MPhil or MPP would be fine but the MPhil might open more doors through name recognition.
Alternatively, there are a few master’s programs out there that really focus on tech-ing people up. For example, USF has an applied economics and an IDE program that are well-regarded. A bunch of master’s programs are trying to distinguish themselves on quantitative skills and I don’t expect all of them to require an econ background, so maybe it’s worth looking around more.
Even working for a year and applying to more quantitative programs saves you a year over doing two masters.