"World governments have committed to halting human-induced extinctions and safeguarding important sites for biodiversity by 2020, but the financial costs of meeting these targets are largely unknown. We estimate the cost of reducing the extinction risk of all globally threatened bird species ... to be U.S. $0.875 to $1.23 billion annually over the next decade, of which 12% is currently funded. Incorporating threatened nonavian species increases this total to U.S. $3.41 to $4.76 billion annually. We estimate that protecting and effectively managing all terrestrial sites of global avian conservation significance (11,731 Important Bird Areas) would cost U.S. $65.1 billion annually. Adding sites for other taxa increases this to U.S. $76.1 billion annually. Meeting these targets will require conservation funding to increase by at least an order of magnitude."I'll leave the details of their methodology to the article, but basically it uses a combination of expert estimates of conservation costs, and then using them as a basis for modeling that includes information on forests, breeding, and size of local economies.
The estimate surprised me a bit, because it's more-or-less one-tenth of 1% of the global economy--a very large amount, but not an unthinkably large amount. However, my guess is that the practical issues of protecting and managing biodiversity-protection areas may be much larger than the straight monetary cost implies.
For those who want some additional discussion of biodiversity issues, the journal Wildlife Research has a recent issue with eight articles on "Prioritisation and Evaluation of Biodiversity Projects" that seek in various ways to tackle the "Noah's Ark" problem--that is, if the world isn't going to do all of what it could to conserve biodiversity, how should priorities be set? And here's are some summary statistics from the IUCN [International Union for the Conservation of Nature] Red List of Endangered Species.