The 2010 Gauteng Freeway Improvement program upgraded or expanded 185Km of Highways, most of the work involved adding extra lanes, and reconditioning of existing road surfaces.
For this exercise we will assume all these roads were built from scratch.
The world Bank reported in 2000 that the average price per Km to build a paved road was $ 886,000, some research shows that costs have actually decreased since then due to new construction methods and materials, but we will use that number to start with.
In 2010 1USD = ~R7.50
At 2010 exchange rates that would equate to about R6.6M per Km of road.
For the 185Km of roads times an average of 4 lanes we could roughly say it should have cost.
185 x 4 lanes = 740km
740 * 6,600,000 = 4,884,000,000 (That's 4.8 Billion)
Let's take inflation and other additional costs into account and say that despite the low cost of labour in South Africa, it should have cost R7bn.
In the end due to interest it will cost R89.722bn, more than ten times as much as it should have.
That's enough, according to road building cost data from more than 40 countries, published by the World Bank. To build an estimated 16,300 km of road (the length Sanrals entire road network), or 3,400 km of 4 lane highway.
They could have rebuilt Gautengs entire National Road (Highway) Infrastructure of 470km 7 times.