MTN have a clause in their terms and conditions that if you use their network to bypass their services then they will charge you a premium rate for data which is about R23/M. They do this for Skype and have threatened to do it for MXiT but have not implemented it AFAIK for MXiT. Even so, with your dubious maths and mine added on, you'd still get about 100 MXiT messages per SMS.