Given the current rate of innovation and progress in construction (and design) technologies, we tend to assume that anything can be built. Think up any ridiculous shape or size you'd like for a building, and if there is a rising economy of plutocrats willing to pay for it, a totalitarian state to muscle it past any potential construction hiccups, and plenty of marginalized laborers to be exploited to build it, there's a good chance it will be realized. Well, you decide you want to go the post-modern route, in particular, millennial OMA, say a perfect sphere, monstrous in size--140km (87 miles) in diameter--and which doubles as the most destructive weapon in the universe, with enough firepower to vaporize entire planets at will? How much would it cost to build this megastructure-to-end-all-megastructures? And, more importantly, how much steel would be required to construct your very own Death Star?
Those are the questions that economics students at Lehigh University wanted answered. Posting their findings on their blog Centives, the students began by estimating the colossal mass of the Death Star and the amount of steel that would go into its construction. Comparing the density of steel in the Death Star to that of a modern warship ("After all, they're both essentially floating weapons platforms so that seems reasonable"), they calculated that the amount would hover around 1.08x1015 tonnes of steel, the production of which would take 833,315 years to accomplish. At today's steel prices, the endeavor would cost $852,000,000,000,000,000, or "roughly 13,000 times the world's GDP."
Still undeterred? While the iron in the Earth could easily provide the raw material needed for one of the upwards two billion Death Stars the students claim could be assembled from terrestrial resources, the recovery of that material would necessitate extent mining excursions to the planet's core, something "we would all really rather you didn't remove." Then there's the off-chance that your fellow mortals or even alien species eventually takes notice of what you're up to and actually tries to stop (or assume control of) the enterprise, in which case you'd have a hard time perpetually fighting them off for 800 millenia.