Well one major investment in early American was financed by private money. The Erie Canal. Lowered transportation costs from Western New York and border states by 90%, transformed America as products moved down to Albany and NYC and people, farm equipment and so forth moved up river to what was then called the Northwest.
There have been a few good federal investments but its been a mighty long time. The Alaska territory was a good investment. The Louisiana purchase was a good investment. The Louis and Clark expedition was a good investment.
The thing is that all of those things were done and the government effectively stepped out of the way and let private interests take the lead. These days the government never gets out of the way.
NASA could be a great investment as a pathfinding and prospecting agency but international treaties eliminate any possibility of real profit in space.