The economics geeks over at Centives did some calculations, and based on just the cost of the steel required to build a structure of that size, they figure it would cost $852,000,000,000,000,000. "Or roughly 13,000 times the world's GDP. But then again, you can just take out a loan from the entire planet and then default on them in the most awesome way possible."
Of course, this raises some questions — like, is the Death Star really made of steel? Or some other alloy from that far-away galaxy? Also, wouldn't people with the ability to build something like that also have super-advanced methods for mining and processing metal, especially if they don't mind destroying a planet or two along the way? Finally, wouldn't the expensive part of the Death Star be that famous reactor core? [Centives]