This is a fascinating area and I agree with Theo that making meaningful
comparisons is difficult.
Here is an interesting way to look at a comparison between Germany and
India. As I understand it, the top 80 million people in India have
about the same amount of income/wealth as the Germans (of whom there are
about 80 million). It's just that in India there more than a billion
more people beyond that who live on a lot less.
While such comparisons are interesting and might be useful in terms of
at least some internal motivations, I think there's an emotional risk,
since issues around money can be so emotional.
Still, if we're going to do them, because they are fun and interesting,
one other factor to include would be per capita readership. I suspect
that most of the difference between Germany and the US in terms of
donation levels can be explained by the higher per capita readership of
Wikipedia in German.
--Jimbo