I must admit I don't understand what this means? So Sasol could provide a replacement for oil that only costs 10-20$ per barrel? Why isn't everybody using their stuff now, instead of the 70$ "real" oil?
Also, in my mind "peak oil" was actually just "peak fossile fuels" - how much more coal and gas is there than oil?
"So Sasol could provide a replacement for oil that only costs 10-20$ per barrel?"
With sufficient natural gas feed stocks, yes.
As to why it's not being used now, until the very recent fracking revolution the supply of gas had been pretty tight, at least at where it's needed for consumption. See e.g. Russia's regular fights with the Ukraine.
Probably the only places where it might make sense to build GTL plants are in relatively unstable oil producing nations that are currently flaring off their gas instead of doing anything else.
There's also the issue of your confidence in predicting the future. These are expensive plants, and if e.g. a world-wide recession eventually leads to a crash in demand then you may not be able to service your debt on what you're able to sell your product at (there are plenty of people who suspect oil might crash to $10 a barrel, e.g. what if the PRC's demand sharply drops?).
It just means that it's profitable for them as a company to make it at that price, but their volumes are still relatively tiny compared to the overall fuel market. Presumably were we all to switch to gas-to-liquids to increased demand on liquid natural gas supplies would push the price up quite significantly. So it's not likely that we could replace oil with a $20 a barrel substitute, but it's plausible that coal-to-liquids and gas-to-liquids could serve as relatively affordable substitutes even on massive scales.
And I'm not sure how much gas there is, but there's enough coal to last for 50-140 years, depending on demand.
Also, in my mind "peak oil" was actually just "peak fossile fuels" - how much more coal and gas is there than oil?