External costs of coal have been quantified by ExternE.

http://www.world-nuclear.org/info/inf68.html

The report of ExternE, a major European study of the external costs of various fuel cycles, focusing on coal and nuclear, was released in 2001 [...] The external costs are defined as those actually incurred in relation to health and the environment and quantifiable but not built into the cost of the electricity to the consumer and therefore which are borne by society at large. They include particularly the effects of air pollution on human health, crop yields and buildings, as well as occupational disease and accidents. [...] The report shows that in clear cash terms nuclear energy incurs about one tenth of the costs of coal. Nuclear energy averages under 0.4 euro cents/kWh (0.2-0.7), less than hydro, coal is over 4.0 cents (2-10 cent averages in different countries), gas ranges 1-4 cents and only wind shows up better than nuclear, at 0.05-0.25 cents/kWh average.

The EU cost of electricity generation without these external costs averages about 4 cents/kWh. If these external costs were in fact included, the EU price of electricity from coal would double and that from gas would increase around 30%. A summary plus access to more recent work is on ExternE web site. http://www.externe.info