Cost of Healthcare.gov: $634 million

Saethan writes: Healthcare.gov, the site to be used by people in 36 states to get insurance as part of the Affordable Care Act, has apparently cost the US Government $634 million. Not only is this more than Facebook spent during its first 6 years in operation, it is also over $500 million above what the original estimate was: $93.7 million. Why, in a country with some of the best web development companies in the world, has this website, which is poor quality at best, cost so much?