However, we also need to factor in human psychology. It would inspire billions of people worldwide to see a person on Mars. This is not something that can be replicated by sending robotic missions. It's possible that this could increase public support for funding space exploration, and drive more people to go into careers in science.
It's hard to predict the benefits of human exploration of space because we don't know how the world will react. But it's a lot more significant than just measuring the scientific output of the mission.