Complete ripe off on end of contract rates

My sim only contract recently expired. I have not had the time to upgrade yet. so from a R269 - MTN My Choice 20G contract, they are charging me R946 as i have not upgraded yet... i looked through their pamphlet and their MTN My Choice Month to Month contracts are going for R259... so how in heavens name can they change me R946. This is a complete ripe off and dishonesty.. Now how can one want to upgrade if this is the case. Sad thing is that i hardly use the sim 'cos we have wifi at home and only use it when we go out..