|Earth - a super-valuable planet.|
|Mars - no so valuable, perhaps because it's named after the god of war?|
|Venus - almost worthless. I call dibs.|
Personally, I think this exercise is more than a little silly, since Earth's total value to the total human race at present should be infinite (as without Earth, we'd be extinct). However, it's a useful reminder that this little mud ball planet, floating around in a vast expanse of nothingness in an insignificant corner of the universe, is all we have - and that we should probably treat it with more love and care than we currently do.
Thanks to Wikipedia for the pictures.