Roaming Rip-off

I recently returned from holiday in Thailand and Vietnam. I received a bill from Vodacom for more than R11,000 for data roaming. At first I thought that I somehow used vast amounts of data, only to find that the bill was for 90Mb of data (yes Mb, not Gb!!!) at R128/Mb. I normally pay R249 for 1Gb which amounts to 25c/Mb. How does Vodacom justify a mark-up of 514% per Mb of data when roaming?!