Billed me R1200 for 3GB data usage, NO Warning!

We opened a Data contract, with 5 days prorata. We were told that it would cost R166 for the prorata. So we used the data available while Vodacom did not warn us that after 500MB they will charge open ended rates and the sales lady did not relate this either. We used 3GB, which is less than the 5GB we receive per month. The 5GB cost R300 per month. Vodacom then billed us R1200 for the extra 3GB! It is absolutely a *******. That's R408 per GB. Normally it costs R300 for an extra 3GB. Spoke to Vodacom over the phone and they just said they can do nothing. Went back to the Vodacom shop in Tygervalley mall and they said that they will get back to us within 24 hrs. Well it has been 2 days and still no answer. I had to phone them again! Our account went into the red because we didn't budget for an extra R1200! It is crazy and I will never refer anyone to Vodacom!