Read this interesting blog ‘the road to a true price’ by Reinier de Adelhart Toorop and Eise van Maanen from True Price (in Dutch): http://nederlandvoedselland.nl/artikel/op-weg-naar-de-echte-prijs
The content, in short: As far as sustainability policy goes; what is counted, counts. For example: meat tax, flight tax, payment for the use instead of ownership of a car. To be able to achieve this, calculations are necessary. The ‘true price’ can help with this; what would the true price be if all natural and social externalities were taken into account.