High data bill - no credible explanation

I have a data contract with my highest bill previously R579.71. Last month my bill was in excess of R4200. My usage was higher than normal but not enough to warrant such a high premium. Feedback to the enquiry was that the rates increased from 29c to 99c per MB. This was not communicated to me at any stage and what gives MTN the right to implement a 241% increase less than a year into a contract???? Surely I cannot be the only person having this problem. Feedback from the branches is that MTN has the right to implement an increase when they see fit and I basically have to accept it. Your call centres remain unanswered and no one at Head Office appears willing to help, while I am now R4200 out of pocket!