In order to estimate the life cycle expense of the weapon system acquisition from the first demand planning phase until the last disuse phase, we apply the reliable expense analysis method to the current situation in Korea by using the sensitivity analysis. We estimate the total cost of the weapon system acquisition by focusing on the most important variables such as exchange rate, and hence by strengthening the function of the existing expense analysis method, which is used to be simply recognized as a tool of cost reduction. Therefore, this study could make a contribution to the literature in this area, in terms of deriving the most cost-efficient method of acquisition by comparing the various estimated costs that result from the different alternatives of the weapon system acquisition.