Guitar Hero: Aerosmith

I would suggest setting-up another band and earning the $4000 in that, on Medium that is about 6 songs 5starred. This way you can buy the songs, but with your main band you still have the money to save and buy other things such as guitars and characters.

This technique can be used for buying other items.

There are a total of 11 extra songs that you purchase from the vault, here's a list;
• Combination
• Joe Perry Guitar Battle
• Kings & Queens
• Let The Music Do The Talking
• Mercy
• Pandora's Box
• Pink
• Rats In The Cellar
• Shakin' In My Cage
• Talk Talkin'
• Walk This Way

You can use that list to cross reference that you do have all the bonus songs, however they also get a big "purchased" stamped across them in the vault when you buy them.

Now it's a good idea to make use of a small in game exploit. Once you've saved up a whole bunch of cash go to the vault and buy all the items you need for the achievement you want to unlock, you need to make sure you have enough money for the achievement you're going for! Once you've purchased everything and the achievement has popped use the guide button to dashboard the game, doing so will prevent the game from saving which will mean when you boot it back up not only will you have the achievement you just got but you'll also have all the money you just spent unlocking it again and you're free to use that money to purchase the items needed for one of the other unlockable based achievements!

Finish the whole career and you'll only have to purchase one song, Pandora's Box for $500, and you can just dashboard (Exit to dashboard, do Not save)