Eco-costs are so-called 'external costs',  i.e. environmental costs for our society, that are not part of the market price. You could see them as hidden obligations, basically hidden costs that can be avoided by technical prevention measures.
Eco-costs is a measure to express the amount of this environmental burden of a product on the basis of prevention of that burden. These are the (marginal) costs which should be made to reduce the environmental pollution and materials depletion in our world to a level which is in line with the carrying capacity of our earth.
More detailed definition and information can be found here.