The cost may well be around 10c/mb. But this surely is not the point. Does Joe Public care what the cost is? Nope.
Fact is the 8ta and CellC(to a lesser extent) data networks are under utilised, not running at capacity, unlike the Vodacom network. So it's all a game of market share, if you want customers to sit up, take notice and subscribe, you have to offer very keenly priced promotions, like 8ta's 10GB/month for R199. The network with the least traffic will probably always offer the best promotions. I guess it's better to have some monies coming in on your investment, than nothing!
Under 10c/mb may not be sustainable, but if market share is what you after then it's justifiable!