Vodacom top up bills increase

I have a Vodacom contract for many years now and currently are on a top up option. This means that you pay the same fixed amount every month. All of a sudden, the amount is different every month with more or less R50! When I called and talked to a consultant, she said if the company dont have enough income, they may increase the amount??? I hear a lot of customers complaining about this and we did not even got a notification about the "increase"!!!