Check for the Hotel Formula 1 near JNB. It should (tightly) take 3 people and it was about $125 a night max during the World Cup. I stayed at the one near Sandton and it's a lot like a Microtel here in the USA. Nothing fancy just a place to sleep and shower - really tiny with a double bed, bunk bed and a small flat screen TV. It was relatively isolated and secure for JNB with a gate around the premises and 24 hour attendance at reception. I prepaid $54 a night during the Cup and it was great value compared to the other hotels with very limited availability. There is one that must be less than 2 miles from JNB. I saw it as we were on the way out. It's an ACCOR hotel brand btw. You can get the direct site thru google.
If it's after the World Cup availability probably won't be an issue and the price might make 2 rooms doable. If you do stay in the airport in the winter bring a coat. It was freezing the other night. Not sure if there isn't heat or if it was out of service.