Hi Sara,
I did this trip in October last year - there is a review on the review page although this does not directly address the issue of how much cash.
Having done a quick check I took £700.00 in rand and bought around £250.00 back.
I prefer to use cash so there was no card spending.
Lunches were often timed to co-incide with driver rest stops and would often be a sandwich and a drink at prices comparable to the UK.
Other expenses were cash tips to the TM/drivers plus whatever you wanted to spend on drinks at dinner / in the evening. I am teetotal post Covid so stuck to non alcoholic but most people had a beer or G&T or two - prices again much as in the UK. there were only 8 of us so no late partying! There are souvenir stands everywhere and prices were not excessive. Amongst other things I bought a print along the way which cost a lot more to get framed when I got home!
There is high end shopping at J'burg airport and I did spend quite a bit there!
Otherwise is is coffees etc here and there and it is easy to forget how much these cost when buying out!
It is a great trip - enjoy.
P.S. get a SA adaptor before you go - SA fitting is unique to SA.