Living with a high standard of living in a poor city is not necessarily inexpensive. While labour and land will be significantly cheaper, importing western luxuries will likely be quite expensive, since there's far less of a market for those kinds of goods. And that's not to mention potential for corruption, extreme tarrifs etc (but I know nothing of Angola in particular, so I'll stay away from jumping to conclusions there).

Oil happened there. It is the second largest producer of oil and is one of the largest supplier's of oil to China.

Luanda has been notably expensive for a long time, for western lifestyles anyway. I think the main issue there is that the western contingent is small and very corporate.