His plan isn't to land on Mars. His plan is to settle Mars. At the end of the day, it means that humanity will have a second planet to call home. If you have difficulty calculating the value of that, it's because it's incalculable.

(Of course in the process of landing a million people on Mars, he'll need to balance the books to make it financially sustainable. I do have some questions about how he'll do that, but can envision a handful of ways to make it work. The actual value, however, is almost entirely off-book.)