The initial purchase of iRacing is a membership (1 month, 3 months, 1 or 2 years) and includes 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/
Licenses for additional cars are (one time) $11.95, and tracks are either (also one time) $11.95 or $14.95, depending on the track. We do this so that you can pick and choose which cars and tracks to licence to the account 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/