First, it's a study commissioned by, not conducted by Greenpeace (it was conducted by this NGO). Second, if they don't have the resources to run the calculations, they can still check the literature. Third, having checked the study again, it seems they did so: see chapter III section D b) resp. Abbildung 17) on page 71, which is a table with different estimates (including ExternE). In that section, they note the order of magnitude range of different estimates. What I deemed an estimate that seems very rudimentary is in chapter II on page 23, and uses only the "best estimate" for a cost per kWh derived from the studies in chapter III, to calulate the external costs for the entire period of 1970-2008   now this is rudimentary. (The figures are 7.9 cent/kWh for "brown coal" and 6.4 cent/kWh for "stone coal", in terms of kWh of electricity produced.)

_{*Lunatic*, n.
One whose delusions are out of fashion.}