Agreed, there's no doubt it will come down. Seacom will be right there at JINX, any of the ISPs would just need to get their act together and link to it and the floodgates would open.
My prediction: a price drop of 50% on the international component initially. We know that it costs around R10/GB for local traffic from most providers, so that part won't change. Axxess charges R59/GB for shaped so I'd say we'll see R35/GB instead. Decent savings.
Mobile providers are charging R200/GB, so the international component of their pricing is far less. For them it would be the same R24/GB reduction in input costs, which I doubt they'd bother passing on to the consumer.