It really varies greatly depending on where you work (even within the US) and what organization you work for.
Generally, MPA/MPPs end up mostly in the public or nonprofit sector. For US salaries of employees in these sectors, you can see this and this (chart below).
But if you work for an organization such as USAID, for example, you can expect to be paid roughly $10K-25K more than any employer in the US. This is because you'll typically be employed abroad. Any of USAID's major contractors such as IRD, TetraTech, DAI, Counterpart, etc. also pay very well. If you live overseas, you also generally get housing, security, meals, transportation and fully- or pratially-paid R&R.
If you are lucky enough to end up in the UN and work overseas, you get similar royal treatment. The UN also pays you thousands of dollars in "hazard pay" on top of your salary if you work in places like Afghanistan, Iraq, Somalia, CAR, South Sudan, etc. (or anywhere else that the UN considers "hazard" deployment). Some UN employees get two-week R&Rs for each six weeks worked.
Additional degree in computer science is a plus, but in my experience not a major plus factor in the industry I described above. In this field, you need program management, paper chasing, multicultural experience, etc.
Hope this helps.
---
Chart below from MPADegree .org
Chart below from