Hmmm its tricky ; as an example and correct me if im wrong;
If you could buy 37500 miles it would cost you £393 approx this would give you enough miles for a cash plus miles return in C to South Africa;
so £393 plus the cas element = £648....then of course there is the tax approx £500 so £1148 total; but then no miles earned; still its not bad depending on the cost of a purchased ticket at the time