Hi Bronwen,
I can imagine how stressful this must be for you. When I look back at medical bills we had in 2012 there's just no way that we could've managed a birth in a private hospital without a medical aid.
I understand your concern about government hospitals, but just keep in mind that most moms in our country has to follow this route. I firmly believe that you get better & worse government hospitals, I'd look into the major government hospital in your closest city. The old Joburg Gen (Carlotte Maxeke hospital now) for instance, for long had the reputation of the best cardiac unit in Gauteng, despite being a government facility.
Remember that, apart from the R21k for the hospital, there will be costs for the delivery that gets paid to the gynae or doctor doing the actual delivery, this will easily be R10k. In a private hospital you can probably work on at least R35k for everything. Ask yourself if you are in a position to afford and/or repay this. If you're going to be exceeding your budget or stressing yourself by too much, then perhaps you'll have to walk the government route.