For a system of this size, It's expensive. I agree with GP, $600 million is pretty cheap for a system intended to serviced over 100,000,000 people. Less than $6 a user is a pretty good deal.
Perhaps that is the first problem. Perhaps they should do a better job of estimating how many people will actually use it. There are only 100,000,000 households in America. Roughly half can't use the website, as their state has their own site. And then how many of these people also have insurance through their work? Throw in the fact that the users will use it over several months, and will most likely only use it a few times, and then never again... $6 per user is pretty excessive.