So the price tag is about $100 billion. Will the contributions the space station makes to Science and society at large be sufficient to provide a $4 billion/year real rate of return in perpetuity, give or take? Does it even come close? If not, it's a net loss to the economy and inhibits economic growth, and there's a bunch of people who'd like to talk to their politicians about that come Tuesday.
It's a certainty that if we're ever to leave this spheroid, we need to build things like the ISS to learn how to live in microgravity environments with limited earth interaction for long periods of time. We won't know if the ISS paid off for a while, but at least it's a project which is forward looking and leads farther along the roads that we need to travel in the future.
While we certainly need to look for ROI in the projects that the government funds, the ROI for many things that the government needs to do is very difficult to measure. Consider how difficult it is to measure the relative importance and ROI of education versus infrastructure expendatures. We know for sure that they affect GDP, but it's hard to say how much, and which schools or projects do it better?