Over charged for data

I bought the 1GB Nite @ R12.50 (use between 1am to 7am). And when I got my bill it stated that I paid R42.98 for the 1GB nite data. I called customer care to tell them about the mistake. The woman over the phone just said I was mistaken and the bill is right. I asked her to put me through to someone who might understand that its not the right price to charge R42.98 for 1GB nite data and she just hung up. How can cell C show me they will only charge R12.50 and then bill me at R42.98 ?