Mtn doesn't explain pro rated charges

I was recently eligible for an upgrade and decided a month to month data contract would be the way to go. I made the intial payment in store and then was informed I would be prorated. The package I signed up for was R299 a month. I receive my bill and I see I'm being charged over R800. Those charges don't make sense to me. I phoned retentions and they offered me a cheaper deal for 24 months. Firstly I wasn't explained I would be charged almost triple the price initially. The detailed invoice showed a charge for Feb, and when I enquired about it, they said it's a misprint and the charges are for December. According to my knowledge, I paid a prorated charge I December, and I was only given 20 gigs(was a deal for 70 gigs) I'm not happy with the resolution cause the charges were not explained to me properly and now I'm out of pocket. Mtn is over priced and I'm not a happy customer. I will never recommend them to anyone.