Roberto, I’m not implying that it would be simple. Quite the contrary, it’s a complex matter. But not less complex than estimating the environmental impact of a nuclear plant. At the moment I’m thinking that some indicators can be gathered from OLOH reportings (contributed lines of code and man/month to existing projects). Other indicators can be donations to relevant players, like FSF or SFLC or OIN. I’m confident there is a way to create such index.