GOVERNMENT GAZETTE No. 44159, dated  February 13, 2021 says, among other pieces of information the following:

 (c) All international travellers arriving at the airports listed in paragraph (a) must provide a valid certificate of a negative COVID -19 test, recognised by the World Health Organisation, which was obtained not more than 72 hours before the date of travel.

(d) In the event of the traveller's failure to submit a certificate as proof of a negative COVID -19 test, the traveller will be required to do an antigen test on arrival at his or her own cost and in the event of a traveller testing positive for COVID -19, he or she will be required to isolate him or herself at his or her own cost, for a period of 10 days

The antigen test on O.R. Tambo airport cost R150-170, i.e. $10-12.

Full text of the gazette is here: files/travel_rsa.pdf