The big long-term money will probably be in raw materials and power, although specifics like this have a way of surprising us.

Just asteroid resources alone would be worth it once the technology is mature enough.

Here's a new link to my article on the subject. Keep in mind I wrote it over 30 years ago, and the numbers would be different now.

A proven resource with big demand will command large investments with long-range returns.

Look at oil and gas.

U.S. companies alone spend over $50 billion a year on exploration and development.

The Prelude floating gas platform being built by Shell will cost ten billion dollars.

NASA's annual budget is about $20 billion.

Depending on the source, the entire Apollo program cost between $105 and $170 billion current dollars.

Find a way to make money out there, and give people any kind of technology to work with, and they will go. I've spent most of my life trying to help bring this technology into reality.

El Dorado is out there waiting, with no endangered species, no ecologies, and no natives.

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By the way, I don't see the Age of Exploration as an economic failure.

Recommended Wikipedia article here (usual Wiki caveats), along with the links right above it.