Mtn data very expense!

I am a fairly new customer to mtn (3months been using it), but seems like the more I buy data they become more expensive and I would qualify for 1 gig for R10 a day but as soon as I start purchasing that gig for studying purposes after 3days or so it's not available anymore.

Same thing when i used it for 1 month I would buy 1gig for R40 for a day but the more I buy it the more I don't get such offers anymore this is so frustrating honestly!

Someone please advice which plan is suitable for students that are studying.