Usage limit increase.

Mtn deducted R2500 from my mobile contract which i usually pay R868.00 pm. Apparently i increased my data usage limit to R1180 of which the assistant at MTN told me its R2 per 2MB. How can we be charged at such high rates?? . I feel like MTN has ripped me off because i wasn't made aware of such until i have to pay the amount. Please be aware of the usage limit increase.