Forza Motorsport 4

To earn this achievement you are required to have the 5 most expensive cars from the game disc 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. Aston Martin 1958 DBR1: 6,000,000CR
2. Ferrari 1957 250 TR* - 8,000,000CR
3. Ferrari 1964 250 GTO* - 10,000,000CR
4. Ferrari 1967 330 P4* - 9,000,000CR (or level 30 reward)
5. Shelby 1965 Cobra Daytona Coupe: 5,000,000CR (or level 30 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 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: 1 of the 5 cars can be obtained for "free" when you reach level 30. Every time you level up you'll generally have the option to select a free car from a few options. When given your choice of car at level 30 I recommend picking the "1967 Ferrari 330 P4". This is the second most expensive cars in the game (so it will save you a good chunk of cash). The Shelby Daytona Coupe is also available at level 30 but the Ferrari is a better choice as it works towards the "Ferrari Collector" achievement as well.
UPDATE: Runner7922 suggested picking up the Shelby Daytona Coupe as the level reward instead as the Auction house is typically flooded with the Ferrari 330 P4 allowing you to pick it up cheaper than than the Daytona.

Use the Auction House: All of these cars can be bought though the auction house, and in general they will cost you substantially less than 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 tip 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 these cars will cost you 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 3 of the 5 cars in needed for this achievement are also needed for the "Ferrari Collector" 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 "Ferrari Collector"

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

These are the 5 cars you need with the credit requirements:

1964 Ferrari 250 GTO: 10,000,000 CR
1967 Ferrari 330 P4: 9,000,000 CR
1957 Ferrari 250 Testa Rossa: 8,000,000 CR
1958 Aston Martin DBR1: 6,000,000 CR
1965 Shelby Cobra Daytona Coupe: 5,000,000 CR

TOTAL: 38,000,000 CR


Alternatively, you can get them for 3 Car Tokens each. 15 tokens will net you all of the cars, which comes to $12 (13 tokens for 800MSP + 2 tokens for 80MSP each).

Also, you can get either the Ferrari 330 P4 or the Shelby Cobra Daytona as a reward car for reaching Level 30. I recommend choosing the Ferrari 330 P4 since it costs more and it will also go towards the Ferrari Collector achievement.