Actually nowadays, transit costs very little since we have all these cables competing with each other.
The real cost now lies with the access network, eg. Octotel, Vumatel ect. They have the expensive networks to build and maintain. NapAfrica JHB have brought in most of the content, eg. Akamai, Google Content, Netflix Cache ect. That shifted a lot of international traffic to free peering at NapAfrica.
Trust me, the expensive part is the FTTH networks. The other expensive thing, is IPC. IPC is expensive cost to the ISP. Where the access network, is expensive to the access network in question. Eg. Most notably here, Octotel is running congested in many places (at least that is how it seems).