MTN Contract data purchase chaos

I do have R450 of airtime on my contract cell phone and 0 data. The option of purchasing that for Monthly of which 500 MB costs R99 it says i have insufficient airtime. I even tried daily option but it gives me the same message. So the only option that is allowing MTN contract users to buy Internet Services data is a One Time option that has a 6 months or 12 months options which the service is hardly available which gives error message if not the pricing for purchasing the data are very unrealistic (i.e. R340-390 for 500 MB) compare to networks such as Vodacom (R99 for 500 MB), Cell c or Telkom mobile (R70-90 for R500 MB) that provide reasonable pricing for Data including their Contract clients.