Well but how about a cutting edge silicon fab that gave the entire world a powerful device that fits in one's pocket to access all of human knowledge, instantaneously? For 20B? Add to this the fact that technology is powered by science and you get a very different picture.
Why is it I generally only ever hear this argument (or some variant of it, e.g. starving children) when discussing a proposed scientific endeavour? Why not trim the US military budget by 10% and generate 3x as much revenue, you could house 1.2 million homeless for that price.
With $20 billion, you could house 400,000 homeless people for three or four years, not forever. And that's assuming they need only rent and not mental health or other services.