Cars and tracks are 1-time purchases.
Once you paid, you can use them (provided your subsciption is running, your internet on and your license at the correct level if you intend to race the cars) - even if you stop subscribing for a while your purchases are still there when you take the plunge again.
You get discounts for emptying your wallet (6 items) and purchasing all the content there is to purchase (on new items).
You can also get credits to purchase cars and tracks, without paying: participation in series, referals, promo codes / renewal bonuses.