> There is no multi-billion dollar return for putting people on Mars.
Actually, there almost was, in the 1980's. A major TV network came to Boeing, where I worked at the time, and asked us how much for a Mars mission. We worked out a mission concept, and gave them an estimate. The network figured they could sell as much advertising for the mission as an Olympic games, but spread over a couple of years. Unfortunately the mission cost we came up with was twice the ad revenue, so it died at that point.
If you can boost the revenue with things like "Mars, the video game", and "Mars, the branded merchandise", and cut costs with modern technology, you *might* get it within reach of break-even. Throw in some government-funded science payloads, and it might work a profit.
Back then, there were only three TV networks, and they could command a big audience. I'm not sure a network could front that much these days, although they still compete for the Olympics.