My 2-50 rule
My 2-50 rule is that for 2% of the cost, unmanned missions will provide 50% of the science that manned exploration provides.
I've mostly made up my rule out of thin air, but I suspect that if the Lunakhod sample return missions had been scaled up to 2% of the Apollo budget then we'd probably have a decent amount of lunar samples. Not 100s of kilos, but enough.
And Apollo is the very best case for manned science - the other ones provide very little science (yes Raoul, I know they've done some, but not much).