The last selection on the right in the Main Menu, The Marketplace is where you will find all of the add-on items that are available for the game. Its menu includes:
-- Popular Cars
-- Car Manufacturers
-- Car Packs
-- Autovista Cars
-- Track Packs
-- Season Pass
-- Car Tokens
Just like it appears, the first pair of selections on the menu provide a sorted view of the cars that can be added to the game based on their popularity or their manufacturer. When you select either you will only be seeing the applicable cars that are part of the DLC expansion packs (some of which you should already own). The same is true for Manufacturers in that the selections you see are just the cars included in the DLC (whether you own them or not).
If you do not own a car you select in those menus you will be given the opportunity to purchase it in several different ways. For instance, if you do not own the pack that the car is part of, selecting it will cause a purchase menu to pop-up. Using the 1965 Pontiac GTO as an example:
-- Purchase Season Pass - 2,400 Microsoft Points
-- Purchase American Muscle Car Pack - 560 Microsoft Points
-- Purchase Pontiac GTO '65 - 160 Microsoft Points
-- View American Muscle Car Pack
The choice you are being asked to make is a simple one: either buy the Season Pass, which gives you a total of six (6) add-on DLC Car Packs, each containing 10 new cars, to be released monthly from November 2011 through April 2012. A Bonus for this includes the American Muscle Car Pack, so you really get seven (7) Car Packs for your 2,400 Microsoft Points.
Alternatively you can purchase the American Muscle Pack or just the GTO (or other cars) from the pack on a car by car basis.
A Race from Inside the Car
-- Car Packs --
When the game was released the following Car Packs were available:
-- 2012 BMW M5
-- American Muscle Car Pack
-- Launch Bonus Car Pack
If you pre-ordered or the copy of the game you bought was from the first production run, you received a DLC code in the game box for the Launch Bonus Car Pack, and the 2012 BMW M5 Car Pack is a free one, so the only one you can purchase is the American Muscle Pack (and if you have the LCE you got that for free anyway).
-- Track Packs --
At launch the available DLC Track Packs for Forza 4 consisted of just the Test Track Benchmark in Odessa, USA, and the Top Gear Soccer Field in Dunsford, England. If you purchased the game as a pre-order or you bought your game from the first production run you received a DLC Code for this in your game box.
-- Car Tokens --
A controversial addition to the game, Car Tokens are exactly what they sound like: a Token that can be used to purchase any Car from the Buy Car Menu in the game. Understand, this does not include DLC cars, rather it is the cars you ordinarily would be paying in-game credits to obtain. So instead of racing dozens of races to earn the money you need to buy the cars, you can alternatively simply pay Microsoft Points and purchase Tokens, then use the Tokens to buy the car.
Tokens are available in the following counts:
1 Token - 80 Microsoft Points
6 Tokens - 400 Microsoft Points
13 Tokens - 800 Microsoft Points
You may think this is rather a pointless resource since you can earn credits free in the game simply by racing consider this -- a 2011 Lamborghini Sesto Elemento costs 2,500,000 credits, which is a LOT of races. That is 2.5 mil for one car, whereas the same 2.5 mil could buy 15 or 20 other cars, so spending some Microsoft Points to acquire it starts to make more sense... But you should be aware that the tokens are not a 1-token 1-car exchange, rather it depends on the price of the car! A 2.5 mil car might cost 1 token, but a 9 million credit car will cost you 3 tokens. Something to keep in mind.
I mention that this is controversial and it is -- the community is not sure how it feels about this if the chatter on the boards is any indication -- partly because there are Achievements that are associated with owning certain cars, so the feeling is that offering players the ability to use real world money to buy cars translates to selling Achievements. The other side of the coin are players who say that what you are really buying is time, so there is nothing wrong with that. You will have to judge how you feel about it on your own...