In the speech where Donald Trump was declaring his participation in the election, he mentioned a website which is worth 5 billion USD. But which website was he actually mentioning? Here is the YouTube link of his speech. Starting with the relevant quote.
I believe that this fact check covers that speech (or a similar one).
He singled out the healthcare.gov website, which was unveiled in the fall of 2013 with a panoply of bugs and glitches, calling it "the $5 billion website for Obamacare, which never worked. Still doesn't work."
Assuming that that is the right speech, then the website is healthcare.gov and the $5 billion number is disputed. The Obama administration claims $834 million. The fact check claims that the number is around the range of $1 billion to $2 billion.