It’s a funny question that many would answer as “infinite”. But putting a price tag on some of the “ecosystem services” that the planet provides is a good way of understanding exactly how valuable they are, and what it might cost us to replace them. It’s also a way to quantify the costs of business externalities such as pollution. One of Hunter Lovins’ favorite anecdotes: If the $200 Million Biosphere II project wasn’t enough to provide clean air for 8 people, what’s the value of a system that does it daily for 6 Billion?
EarthTrends has a good article with some more details here. The current estimated value of Earth’s ecosystem services: $33 Trillion, or almost double the global GNP.