Knowing the Price of Everything?

There seems to be a potentially interesting line of debate opening up(http://www.guardian.co.uk/environment/2009/feb/09/conservatives-biodiversity) with the UK Conservative party apparently advocating (like Australia, Malaysia and the US) a voluntary scheme of allocating bio-credits (cash values) to every species and habitat in the UK. This move (placing values on animals, plants and their locations) is intended, claim its advocates to reverse the decline in 'biodiversity' (always a difficult thing to measure anyhow) consequent upon development, pollution and climate change. This is notionally timely as the European Union (not always a favourite flavour of that party) have a target of halting biodiversity decline by 2010. I am not sure that voluntary schemes have a good track record in this area but the hope is that business would 'factor in' the costs of what their activities would change and the "ecosystem services" would be able to claim extra credit. If followed, there are many unanswered questions. What is the relative value of a Red squirrel compared to a Bee orchid? Although it is hoped the scheme might discourage developments on rare ecosystems, does the size of the ecosystem and its connectivity change the value? Does a rare species at the limit of its range have any value at all?