Note that Luanda is only outrageously expensive for ex-pats, if you want to live how you are accustomed to in the West.
If you accept the lack of access to Western goods, then it is much more reasonable.
Why is food so much cheaper? Why is land so much cheaper? Why are utilities so much cheaper? What are they doing that makes everything so cheap? IMHO, this is really a phd thesis sized undertaking and would probably teach us a lot about the inefficiencies in our western system.
Have you ever been to one of these places?
People are much poorer, and the standard of living is much lower. Things like utilities often don't exist, or or much less reliable than in richer countries.
Some things are cheaper: branded clothes and foods are generally priced beyond the means of most, so people don't buy them. Transport is cheaper, because people don't own cars - they walk, or bus, or share motor-scooters.
Accommodation is cheaper on a per-person basis, because 6 people in a single bedroom apartment is normal.
A few exceptions exist of course - the US health system costs a lot more than anywhere else in the world given the health outcomes. Places like Chile and Cuba pay a lot less for health care, but sit above and below the US on tables of life expectancy[1]. There are also things like extreme inequality in some countries which can cause other distortions.
But there generally really are no secrets - they live cheaper because they are poorer.
[1] https://en.wikipedia.org/wiki/List_of_countries_by_life_expe...