Contract to increase when I signed a 24 month

I took out a Vodacom Topup MyGig2 contract for R99 for 24 months and now I have been notified that it will increase from 1 May 2015 to R109. My Individual Contract Agreement does not state anything about price increase or cancellation of contracts. I signed the R99 for 24months so what is the point of signing a contract for that amount if it can change. I am not very happy as I have just had problems since I took this contract out.