When we're talking about colonization and/or building infrastructure, "cost-effectiveness" means different things in different time frames. Over 10 years we might invest billions and get nothing back. Over a century we might invest 100's of billions, and get trillions back. The thing about infrastructure is it's like a goose that lays golden eggs, vs. a golden egg. You build it once, and (with proper maintenance) it may keep producing indefinitely.