By any reasonable standard the roughly $400m spent on implementing this was incredibly excessive. If a private company had wanted to build this system for profit, it would have been done for under $100m.
And if it cost that much, it would be!
If, on the other hand, the numbers that your referencing came from... say... a couple journalists incorrect understanding of the total bid cost (based on both delivery and subsequent modifications and years of projected upkeep) and not instead from the actual cost to get what we currently have...
The big mistake of the ACA was that it did not allow for the creation of privately run and owned exchanges.
The ACA didn't need to do anything -- that's the status quo. There's been literally nothing preventing something like that from being set up since the invention of the web.
But hey, since we know that the free market always comes up with the best solution, clearly the lack of such exchanges means that people didn't want something like this, right?