Depending on which mobile network you go to, they will require you to buy different amounts I guess.
First in order to register as what they call a super dealer, you will have to buy something like US$10,000 per month and this will give you a commission of 2 percentage points. Then you are able to resell it to the guys who vend airtime at about 6-8% (depending on what mobile network).
If you sell direct to the customer you will be able to make a greater margin, but then this will be very difficult to push such numbers unless you have some revolutionary way to push the voucher pins (or even if you are going to buy pinless airtime).
If you want to sell direct to the public I'd suggest you get with guys like Mvendr or Easi.co.zw who allow you to store cash in your wallet with them and draw whatever airtime from any network and sell to customers as and when you want
Better yet, they allow you to vend things like ZESA prepaid unit, DSTV, Africom, PowerTel and even TelOne.