The Emperor spent a lot of dough on building the Death Star. It had every amenity an artificial planet could offer: soaking tubs, day spa, and a giant laser cannon. But then it got blown up. As penance, the Emperor ordered Darth Vader to use his own allowance to finance the construction of a second Death Star. So, Vader emptied his piggy banks and . . . well, you know the rest (now you know the real reason why the second Death Star was unfinished – Vader ran out of change)! But did you know that all of Vader’s piggy banks looked just like him? True story.
— ThinkGeek :: Darth Vader Bust Bank