It's not "mistake" singular, it's actually mistakes.

The first mistake is the $500 million rover is fried.

The second mistake is believing the first mistake.

(or put another way, the first mistake cost $500 million, the second mistake which they didn't realise at the time, saved $500 million)

But you can't explain the second mistake without first explaining the first mistake, hence the title.

I guess the idea is that it potentially could have cost $500 million therefore it was a $500 million mistake. It's not exactly accurate but it does help contextualize the gravity of their mistake.

It was feared to be a $500M mistake.