I think that's pretty decent considering the move in the Canadian dollar! They are about $280 usd at tirerack without shipping/duty/conversion.
If it's the current ContiSportContact 5 / ContiSilent Tire, that's a good price. This is the tire you want to compare to: http://www.tirerack.com/tires/tires.jsp?tireMake=Continental&tireModel=ContiSportContact+5&partnum=435WR1SPC5XLCOSI&vehicleSearch=true&fromCompare1=yes&autoMake=Tesla&autoYear=2012&autoModel=Model%20S&autoModClar=P85
Make sure you get a quote for the tires installed. I went through the service centre on Lawrence for one tire...they quoted me $ 460.00 which was reasonable but after labour, fees and taxes the tire was closer to $ 700.00