My (future) father in law sent me this excellent article, where centives.com
asks the essential question: how much it would cost to build the Death Star?
Short answer: $852,000,000,000,000,000 (and 833,315 years).
So it's a pretty hefty capital and time investment, but given the awesome power of destruction it comes with, it's probably worth it. As my fiancée put it: "well, we did build the pyramids, and they don't even do anything". :)