The general idea is that any large ISP will be able to buy bw at much cheaper rates than currently. So, if IS for example changes their accounts from R299 for 5GB to R299 for 20GB, then everyone else must follow suit.
The minumum price of broadband will stay roughly the same. A DSL connection will not go below +- R150 a month, but the GBs will gradually become dirt cheap. A 4 Meg DSL will still cost around R450 a month, but will become high/un-capped.
There are fixed input costs to any broadband connection. I would say around 30% of the cost is in actual national/international bw. The rest is hardware, support and maintenance. So SEACOM will be reducing that 30% cost by 50 times, not the full 100% price of broadband.