Think about it this way, if they truly are the first company to start laying claim to mineral deposits (or provide service to whatever govt does lay claim) on asteroids, satellites like the Moon, and other planets the amount of value in this company - even over the upcoming centuries - is going to be greater than the East India Company, General Electric, or Apple ever where.
Now that's an investment well worth leaving to our kids.
"Having technology able to recover said deposits and deliver them to an area of meaningful economic activity without costing several multiples of the value (or, more realistically, several thousand multiples of the value) of the minerals in question" would certainly justify a very high valuation for SpaceX. But they do not have such technology, and it is far from clear that they are on the path to having that technology anytime soon.
And nothing has much present value today based on its prospective value in centuries. In 100 years, we could be superintelligent nanotech swarms. Or extinct. Or anything in between. Only a fool would place much certainty on the value of a mineral deposit that far out.
Certainly SpaceX has an air of "tool builders" about them -- they'll build the shovels and pick axes of the space economy. As evidenced by their current economic successes, they're banking on the evolution of the industry -- not a quantum leap 100 years hence.
A great (related) scifi novel: The Moon is a Harsh Mistress by Heinlein
If I set up a company tomorrow claiming to have solved cold fusion, I'd argue it was beyond undervalued. Until I deliver on that promise an investment would remain a risk. Not that I wish to demean what SpaceX have done so far - it's fantastic. But rocket science is, well, rocket science. And a lot can go wrong.
Earth orbit is hard in terms of energy costs, but relatively easy in other terms: you don't need to solve the long-term independent life support problem, manufacture things off-planet, or figure out how to do supply lines in space.
Although in this instance, that probe is destined for L1, which is still in Earth orbit. (The Lagrange points are where you are simultaneously in orbit around two bodies - Earth and the sun, here)