The initial purchase of iRacing is a membership (1 month, 3 month, 1 or two year) and includes all of the cars and tracks needed to run all official Rookie series.
You can find a list of the cars and tracks as well as how much each membership costs here: http://www.iracing.com/membership/
Additional cars are $11.95 and tracks are either $11.95 or $14.95, depending on the track - this is a one time purchase. We do this so that you can pick and choose which cars and tracks to buy, and don't end up paying for cars and tracks you may not want or use.
You can also find more information here: http://www.iracing.com/what-comes-with-membership/