"Among his proposals are $300 billion in federal funding over 10 years to construct the transportation and manufacturing infrastructure to develop energy and raw materials in space. The money could fund research to advance solar power technology, the study of asteroids and how to mine them for minerals and construction materials, and even scientific bases on the Moon."

That's a lot of money it's basically twice NASA's budget.
I think it's completely wrong approach.
With one billion added to NASA's current budget (and this money isn't allowed to be sucked out by the ISS vampire). NASA COULD go to the Moon and find water, etc. and be done in less than 5 years (so less than 5 billion total). And for as little as 200 million NASA/govt could buy water payloads. Both of these could do a lot towards getting us into space in a short period of time and cost 1/60th of the above idea.
Now, this doesn't mean that a lot of money wouldn't be spent in space development- it just means the private sector would be spending the money needed for capital costs- starting small and building into a large space economy. In other words after around 10 years or so, 100s of billions per year could spent by private sector.