According to Wikipedia:
"The original budget for CGI was $93.7 million, but this grew to $292 million prior to launch of the website. While estimates that the overall cost for building the website had reached over $500 million prior to launch, the Office of Inspector General released a report finding that the total cost of the HealthCare.gov website had reached $1.7 billion."
https://en.wikipedia.org/wiki/HealthCare.gov
Here's the report claiming $1.7B total cost: https://oig.hhs.gov/oei/reports/oei-03-14-00231.asp