Negotiating salary and benefits in South Africa

It all depends on the level of employment and the company. Expat benefits differ and it's not easy to establish benchmark.
The biggest thing driving the difference is whether the company moves you across, or if you are find a job on a "local" market. In the first case, generally companies pay relocation allowance for approx. 3 months and often provide accommodation and transport. After this period, employees are usually expected to cover their own costs.
Most companies will fully cover pension and medical aid. Considering cost of living and instability of rand, this is a must and I would not consider a job that doesn't provide and include this in a package.
There are some companies that on top of that provide accommodation and car allowance or rent car and a flat for the employee at their cost. Some cover cost of schooling for the children, and even spending allowance for the spouse. Quite frequently the family will also have one trip to home country covered by the company.
This depends on company policies - it is always worth to ask for it, the worse that can happen is that you hear "no".
If company has no special policies as to extra benefits, it is always good to try to negotiate "sign-on bonus" - once off, upfront lump sum that can be high (even as big as annual salary, although it's not a rule). It is then not part of the "expat package" and company can still demonstrate, that there are no preferential treatment between expats and locals (which is often quite important).
In the second case (you coming over to a job that you found yourself) there is NO EXPAT BENEFITS. You are part of local employment force. You will be treated like a South African and you won't be able to change your status.
In either case you should negotiate everything before you sign the contract. You won't get much when you are already here.