It will probably vary depending on the type of mission, but I would imagine (on the average), that for the cost of one manned mission you could send ten different robot probes with ten separate missions to the same destination. For the cost of one extended colony on another world, you could fund a complete robot space program visiting many worlds, even making repeat visits if they turned up something really interesting.

I agree we need a continued but minimal human presence in space, such as the space station, to study space medicine and long term effects of zero-g and radiation, as well as a testbed for technologies useful for future manned missions. But I also think we need to send multiple robot probes, (including landers, if possible) to every planet, major satellite, and interesting asteroid and iceball in the solar system. We also need to start constructing many automated observatories in space, several for every band in the electromagnetic spectrum, including a REALLY big radio telescope and a VERY wide baseline radio interferometer. There's enough out there to keep us busy for a century, all probably for the cost of one hit and run Mars mission or one lunar colony.

I am convinced that with this kind of saturation of solar space with probes, it is only a matter of time before we turn up something that will justify the cost of a manned mission, or a permanent base somewhere, or even a full-bore colonization program. And in the unlikely event we don't find anything worth a closer, long term look...then at least we will have gotten a lot of good science done.

The alternative is that we blow a lot of money on a few big manned missions, turn up nothing of interest, and some future bureaucrat pulls the funding plug on all space activity.