The 2013 NASA's Office of Inspector General (OIG) report, Extending the Operational Life of the International Space Station Until 2024, shows the ISS annual operating costs at $2.9 billion. How much of this cost can be linked to the fact that the ISS has a human crew? It takes a lot to support a human crew in space: more mass to launch in resupply, launches to ferry crew back and forth, life support systems, higher risk monitoring, etc. If it is possible to separate these costs out, I would be very interested in the results.
Edit: Some users have mentioned that estimating this depends on what you count and what you don't. Let's imagine that Robonaut were completely capable of performing solo operations on the ISS, and that Robonaut has now been given full responsibility for performing all on-board activities on the ISS. How would this change the annual operating cost? (Don't try to estimate what it would take to maintain Robonaut at this level of activity--assume current Robonaut costs whether those are lower or higher than they would be in this imaginary scenario.)
Please try not to get hung up on the plausibility of this scenario. I'm not asking for a fully automated ISS--I'm using it as the premise to demonstrate how I want the costs separated.