It mostly depends what games you buy from which publishers. If you mostly buy AAA games from Activision or Ubisoft, then yes the price will be more because they set it to be more. Most likely the strategy they follow is that they believe South Africa does not have enough volume for a low price point to make sense, so they charge more to maximize profits.
Other publishers rather take the strategy of lower the price to hopefully attract more buyers.
On your point of adding up a random selections of 20 - 30 games: that is really subjective and dependent on what games you allow to be selected. If, for example, you allow the whole Steam catalog (including all the trash), then it will most likely come out even or lower. If you exclude games with some sort of filter (maybe has had maximum players of 1000+), then again I would argue that you would come out even or possibly lower, simply because it seems most indie companies price there games around there (as could be seen from examples). If you can only select AAA games, then yes most likely you will come out higher.
If I were to add the last 10 games I bought together, I would actually say that i paid either the same or possibly lower, but it's because I don't normally buy games that are from big publishers (mostly because they are normally repetitive sequels), or that I don't buy expensive games until sales, while I will buy a R300 game on release if I want it.
