Over Priced Contract Deala

Hi, I have been a loyal VODACOM customer for a few years and have always been very happy with the Network. I am now due for my upgrade and was shocked to see how overpriced the IPhone 6s is compared to the other networks. I understand it is a new phone however how is it possible that MTN can offer deals for R559 and R489 for a similar deal VODACOM offer at R650 and R750?? I am now considering cancelling my contract and moving over where the prices are much more competitive. Please advise.