How much an apartment will cost you monthly will vary vastly depending on area and the building's condition. Also, whether the building has a water tank, generator, parking, security etc.
Do you need access to a school? Will you be working in Luanda or outside?
Personally, I always lived in Alvalade in the city because I couldn't deal with the daily commute from Talatona/Luanda Sul etc. a few hours each way at least.
I would say, and this is a wild estimate that you can pay anywhere from USD2000 to USD12,000 monthly depending on area and standards you prepared to put up with.