You are right, not really R399, but rather R350.
The price Microsoft is referring to is $39.99 for the OS if downloaded. This is currently around R350. The R299 was an estimation based on the Rand/Dollar rate at the time they posted the offer.
But the point is, this upgrade offer is only until January, it won't last the whole lifecycle of Windows 8. So Although they did see massive sales so far, I don't think it will go on at this rate, since very few people would be persuaded to buy Windows 8 (which is almost identical to 7) for $199, which they've said will be the normal price.