To compare the scientific return of robots and people, we can compare the results of the only space rock both have visited: the Moon. The six manned Apollo missions brought back much much much more scientific return than all of the contemporary robotic missions. Apollo brought back 382 kg of moon rocks. Three Soviet probes (first one, Luna 16, between Apollo 12 and 14) brought back a total of 326g. While it's a bit facile to claim that's the whole difference, I would say that the difference in scientific return was at least one order of magnitude, if not quite as large as the moon rock numbers.

Now, the Apollo program also cost much more than the robotic missions. If you are willing to invest enough (e.g. Apollo was >1% of US GDP/year for most of the 60's) you can get an enormous amount of scientific return from a manned mission, but robots are useful for budgets that can't cover a manned mission.