Selecting a system for treatment and disposal of municipal solid wastes (i.e., selection of the capacity, location and type of the processes and management programs) is the key factor which determines the cost of the municipal solid waste management applications. In these applications, every process and management program is an individual economic activity, and they cause not only private costs and benefits but also external costs and benefits in different levels. In the current decision making applications, however, the final decisions for the system selection are mostly taken by the decision makers without considering external costs and/or benefits. In this paper, a new cost optimization model approach which theoretically has to give same decision results at every run under the same model conditions was used to determine an appropriate system for treatment and disposal of municipal solid wastes in a large region. Firstly the solutions were obtained for the case that the objective function included only private (internal) costs and benefits. Then, different scenarios that include external costs obtained from the literature were applied and the solutions were compared with the previous ones. Results showed that different final decisions could be obtained for some scenarios at the same model conditions. These differences were analyzed in terms of the total cost of the system, and it was observed that an annual reduction between 1 and 8 Euro/person could be obtained with respect to the first decision. The effect of these external cost-related reductions in the annual total cost of the system was calculated as earnings in the range of 2-13%. On the other hand, a Monte Carlo simulation which was applied for the range of the external costs indicated some meaningful inconsistency between the values of the study and the literature. All these findings refer to need for a new comprehensive decision making application for real external costs of the study area before final decision. In conclusion, it can be said that this deterministic approach might be useful for environmental managers and decision makers in terms of reduction the total cost and the external costs of the system before final decisions.