There are plenty of expansion packs. There are the standard packs, Ultimate Speed Pack, Movie Legends pack, and the NFS Heroes pack, which add 5 cars each and cost $10. Then there is the Terminal Velocity Pack, which adds 5 cars as well as an expansion to Fairhaven City itself, Hughes International Airport.
You can also get an additional car, the Dodge Charger SRT8, by redeeming a code inside a Slim Jim that you can redeem at the Slim Jim website and recieve the car in the game.
I don't understand why this game is so underestimated just because it is similar to Burnout Paradise. If you have read any reviews that state "The game is just like Burnout Paradise", that shouldn't be a bad thing. Burnout Paradise was an amazing game, and it even states that in most of the reviews. Besides, this game has many differences, such as the inclusion of real-world liscensed cars, a Most Wanted List, a different city, and each car has their own events. The graphics are WAY better too.
This game is definetly worth the price. Just note that there is no local multiplayer of any sort. It's all online.