Forza Motorsport 4

To earn this achievement you are required to have all 22 Ferraris from disc 1 in your garage at the same time. The achievement should pop the instant you obtain the last car to complete the set.

These are the cars and their prices:
1. 1998 #12 Risi F333 - 2,200,000CR
2. 2010 #83 F430GT - 1,000,000CR (or level 38 reward)
3. 2010 #89 F430GT - 1,000,000CR
4. 1957 250 TR* - 8,000,000CR
5. 1964 250 GTO* - 10,000,000CR
6. 1967 330 P4* - 9,000,000CR (or level 30 reward)
7. 1999 360 Modena - 90,000CR
8. 1968 365 GTB/4 - 270,000CR (or level 16 reward)
9. 2010 458 Italia - 240,000CR (or level 34 reward)
10. 1991 512 TR - 75,000CR
11. 2006 599 GTB Fiorano - 310,000CR
12. 2011 599 GTO - 370,000CR (or level 36 reward)
13. 2010 599xx - 1,500,000CR
14. 2009 California - 220,000CR
15. 1969 Dino 246 GT - 55,000CR
16. 2002 Enzo - 1,300,000CR
17. 1987 F40 - 380,000CR
18. 2004 F430 - 150,000 (or level 28 reward)
19. 1995 F50 - 400,000 (or level 31 reward)
20. 1957 Ferrari 250 California - 3,000,000CR
21. 2011 FF - 280,000CR
22. 1984 GTO - 320,000CR (or level 25 reward)

Tips for earning:
The Gift Method: The easiest and fastest way to earn the achievement is to find someone who already owns all of these cars, they can send the cars to you as a "gift" once you've downloaded all the gift cars you will earn the achievement. You can gift all the cars back to them, the process takes about 5 or 10 minutes, but it does require a level of trust on the part of the gamer who "owns" the cars. (Sorry, I don't have the cars to gift as I've resold mine).

Make use of the Level Rewards: 8 of the 22 cars can be obtained for "free" by selecting them once you've reached a certain Player Level. Every time you level up you'll generally have the option to select a free car from a few options. Picking a Ferrari from the list above when you see one will save you some cash. I've marked which cars can be chosen as rewards for which levels. The most important is the level 30 upgrade which has the option of the "1967 330 P4". This is one of the most expensive cars in the game (so it will save you a good chunk of cash), and it works towards the "Exclusive Tastes" achievement as well.

Use the Auction House: All of these cars can be bought though the auction house, and in general they will cost you about 1/10th of the price buying them there as opposed to buying them "new". A lot of people will get into bidding wars on these cars so if you'd rather not deal with that headache look for cars with a reasonable buyout price. You might not be able to get them as cheap as you would by bidding but it will still be substantially cheaper than buying "new".

Pool your Resources: If you have a group of friends (or say a boost session) interested in getting this achievement you can split the cost of buying these cars, and then take turns gifting the cars to each other. Obviously the more people you get the lower the cost will be per person, using the level rewards and auction house tips can reduce the costs too.

Farm for Credits: It's a boring grind but you can setup races and use it to pump up your XP and earn Credits faster than playing the game normally. The methods that have been around since Forza 2 still work. The video I've attached shows how to do this. It's for Forza 3 but still works in Forza 4. This method requires the Test Track, which is only availble as part of the "Launch Pack" DLC which if you didn't get with your copy of the game, you can buy in the marketplace. Other tracks could be used, but the test track is best. Also make sure to turn off the auto steering and braking driving aides, if you don't you'll be running into "invisible walls".

Buy Tokens: If you've got more money (in real life) than time you can buy "tokens" in the marketplace and spend them on cars. Tokens cost 80MP a piece and cars will cost anywhere from 1 token to 3 tokens a piece, it's not cheap. Keep in mind that if you buy a car with tokens YOU CANNOT GIFT IT OR RESELL IT. So you wont be able to help anyone else earn the achievement or re-sell all your cars in the auction house. I don't recommend this option but it's an option non-the-less

Other Notes:
*Keep in mind that earning all of the Ferraris will also earn you 3 of the 5 cars required for the "Exclusive Tastes" achievement so keep that in mind if you're going to be pooling your money or having the cars gifted to you from a friend. The cars I've marked with a "*" are the three that overlap with "Exclusive Tastes"

If you have any questions, find any errors in my guide, or have some addition tips Please leave a comment. Thanks for reading :)

Here the list of the Ferrari you get as Level reward (so you won´t have to buy it):

1968 365 GTB/4 - 270,000 (Player Level 16 Reward Car)
1984 GTO - 320,000 (Player Level 25 Reward Car)
2004 F430 - 150,000 (Player Level 28 Reward Car)
1967 330 P4 - 9,000,000 (Player Level 30 Reward Car)
1995 F50 - 400,000 (Player Level 31 Reward Car)
2010 458 Italia - 240,000 (Player Level 34 Reward Car OR FM3 Profile Import)
2011 599 GTO - 370,000 (Player Level 36 Reward Car)
2010 #83 F430GT - 1,000,000 (Player Level 38 Reward Car)