Comair, which is owned by British Airways, has recently launched a low-fare domestic carrier called Kulula.
Tickets can be booked only through their direct call center or via kulula.com.
Right now, Jo'burg to Cape Town is on sale for 299 South African Rand each way (about 375 Rand or $52 USD tax-inclusive). Availablity of this fare in March is still very good.
Service is strictly no-frills, but very friendly. It's only two hours, after all.
[This message has been edited by YYZC2 (edited Jan 22, 2004).]