Based on the current data regarding one of the large combustion plants (LCP) in Poland, there was determined the relationship between the costs of exhaust gas treatment applied as a result of progressive limitation of air emissions of certain pollutants from LCPs in the EU and the costs associated with the environmental effects of emission reduction. The analyses were carried out in consideration of the emission limits for particulate matter (PM), sulphur dioxide (SO2) and nitrogen oxides (NOx) laid down in the EU Directives (LCP and IED) and the BAT conclusions. The costs of exhaust gas treatment were examined as regards the investment costs, at the assumed amortisation period of 20 years, as well as the operating costs including the increase in electricity used for own consumption. The costs per unit mass of a given pollutant reduced due to the exhaust gas treatment installation were compared with relevant environmental fees and the avoided external costs related to the pollutants analysed. There was also performed the analysis of PM, SO2 and NOX concentrations in the areas neighbouring the LCP under the study. The obtained results clearly show that with careful planning for investment expenditures related to the construction of the exhaust gas treatment installation, the avoided external costs are several times higher than the incurred expenses. The benefits of tightening the emission regulations are evident not only in terms of the reduction of the external costs, but also in the considerable decrease of the concentration at ground levels.