Crazy Monthly charges

Firstly 1 star is a huge compliment because it's certainly not deserving. I have 2 contracts with MTN, R299 for each, so my Monthly charge is R598. I upgraded 1 plan and the new cost is R299, I then look at my November bill and its over 1100. I call up the call centre and they say its for pro rata charges, In which world does that make sense, your call centre needs serious development. Pathetic.