I don't know about South Africa but over here in Germany, you can get brandnew SL55 with a very nice rebate and used ones with low mileage...well...cheap!!!
Right now the market for sports cars isn't too good over here and Mercedes has lots of problems with car sales since they had some serious quality issues. Of course the economy over here in Germany doesn't go too well either, too many people are unemployed and too many people hold back with new car buying because they fear they might loose their job in the near future.
Not much different with Porsche: you don't get too much rebate on a new car but look at the depreciation of the "old" 996 and Boxster models, incredible. I just saw an advertised 996 Turbo with around 40000 km only for under 70000 Euro. Incredible.
This is why I actually tell people all the time: never buy a car looking at the resale value because you can end up with some very unpleasant surprises.
I also prefer to lease such cars instead of buying them, it somehow takes a bit away of the depreciation risk, depending on the lease company and the contract.