I would say that I (very naively) thought that it would be easy peasy to rebuild all of healthcare.gov. I was wrong. There are all sorts things that make it much harder than you'd expect in a private sector environment.
I do think that there are probably ways to get this cost even cheaper - however, from a software profiling perspective, moving it down an order of magnitude or two is a huge win and that it's a pretty clear choice between further optimization compared to an order of magnitude change on a VA, immigration, or other system.
[0] I'm not actually sure how exact this number is either - lots of help required from people from all different contracts - so this is an upper bound estimate
By a rough estimate, a fully loaded developer costs at least $200-250k/year, counting salary, benefits and incidental expenses (ie supplies, offices... etc). So a team of 12 people would cost ~$3M/year and have $1M/year left over for all their other business expenses.
The government might end up paying somewhat less per person, but they'd also need additional people to deal with bureaucracy, coordinate with other agencies, deal with state governments and so on, so they likely want more than 12 people running a system like this.