Their debt is in USD.If the price increase is only on the data, I'd suspect that their data costs are in USD and knowing that their profitability is low, they have to compensate the rand's weakness to avoid jeopardising their returns (they have quite a high gearing and little margin).
How much extra? With Vodacom it's 75% of what's left on the contract. Cell C and VM you pay the proportional amount for the device. MTN you pay the device plus one month subscription.You can cancel a cellphone contract with any provider but you will be billed some extra fees.