Upgraded to 10MB line for R699 being charged R774

I upgraded to a 10mb line in august. The sales consultant told me that it will only cost R699 per month. It is now December and I am being charged R774. I logged a dispute and the contact center told me that because I upgraded, I am being charged 774. Neither the sales consult nor the website stated that upgrading would attract a different charge. Please fix this as I am considering cancelling my line and will not renew my 2 cell phone contract with you because of this. I want my money refunded from August till December for the overcharged amount. It's a shame that every time I got a problem with you I have to come here to get it resolved.