This is complicated and will probably always be the area that is most prone to critique. The authors of the report put some effort to explain their methods - se

This is complicated and will probably always be the area that is most prone to critique. The authors of the report put some effort to explain their methods - see p. 3.2.2, Valuation, pages 18-27 of the report. They seem to have used abatement costs (how much for removing the damage?) where available. For other factors like air pollution, estimates on increased health costs, crops, water, increased corrosion, etc. were used. IMHO the most tricky is the part on "opportunity costs" of things like land use - it's hard to put an uncontroversial price tag on the fact that you cannot go hiking on parts of land used by some industries.