If you want some possibly-far-fetched but very high-ROI space investment strategies, have you seen the Phil Metzger et al proposal for rapid bootstrapping of space industry[1][2]? Even if off by orders of magnitude it would still dwarf the industrial output of the world within a few decades.
[1] http://www.philipmetzger.com/blog/affordable-rapid-bootstrap...
[2] http://www.philipmetzger.com/blog/wp-content/uploads/2014/09...