Gold Bricks

You unlock the ability to buy bricks as you go. Bricks are used to unlock special end-of-episode gameplay. You can't use them until you finish all the episode missions anyway, so there's no real reason to hurry to get these.

brick 1 10k
brick 2 15k
brick 3 20k
brick 4 25k
brick 5 30k
brick 6 35k
brick 7 40k
brick 8 45k

I unlocked through brick 7 after finishing all of Episode IV.

Lego Star Wars 2 Walkthrough