$800 pays for roughly one day's worth of work for a senior developer in the US. One thousand lines of working, debugged code (which is presumably what's being paid for) sounds like an awful lot of code for one person to write in a day, if the problem domain has any degree of complexity. Some parts of healthcare.gov could be quite complex, e.g., interfacing to other government systems whose APIs are not well documented.

And I agree with the people who think that 500M lines of code is unlikely. Judging by some of the statistics that were cited in other comments (e.g., "Windows XP was 45 million lines of code"), this may be more lines of code than Microsoft has written since the company was founded (many of the lines of code in XP were originally from NT, etc.).

It costs ~$300K for a senior web developer in the US currently?

The per-day rate that a company charges is not the same as the salary that person makes. So yes, it costs 300K to hire a contractor full time. Not only in the US, but on most parts of Europe too.

I feel a bit dirty for having to explain such simple things. All the armchair pundits here are declaring 'how can they screw up a simple website'. Nobody here (except if there are people reading this who actually worked on this project) knows how hard this project is, and how much integration is required. Personally, if somebody had asked me to make a quote for a project this size a year ago, and going on the information we have now, I would have said that it's impossible to build something this big and this complex within a year. Can't be done. IT systems of tax administrations of small countries that do less than this system have cost 10 years to get to a stable, workable state. How can anyone build this within a year?

Assuming a normal work schedule (weekdays only), with four weeks of vacation, the usual holidays (10) plus paid sick days (5), there are about 225 work days a year. Multiply 800 by 225 and you get $180000, which is a very reasonable yearly cost for a senior developer (it needs to include salary, benefits, office space, equipment, support staff, etc.).