This article presents an example of a procedure for economic estimation of exploitation of a new lignite deposit. This paper is concerned with a way of assigning of the scope of a decisional variable value in the NPV (Net Present Value) formula. This article also takes into consideration the specific of lignite mining in cash flow calculation, taking into account instead of operating costs, the unit costs of mass exploitation. The analysis presented in this article enlarges a modification of sensitivity analysis for investment risk projects' estimation. Mentioned enlarge modification considers the changes analysis of NPV (Net Present Value) under the influence of two parameters. Moreover, the changes influencing on value of NPV as well as the selling price of lignite, unit cost of mass exploitation, discount rate are not expressed in percentage value but in the real values listed in lignite mines. This depicted analysis considers not only one final parameter as NPV but also others important for comprehensive estimation of investment project, as internal rate of return - IRR, modified internal rate of return - MIRR, profitability index - PI, discount rate RONA, economics value added - EVA and period of return. The presented analysis is based on Legnica Zachód lignite deposit, which is considered for exploitation in regard to its large lignite resources, very good value of lignite resources, sufficient geological investigations and non-significant ecological objections. This analysis considering 100 variants discusses the marginal volumes of analyzed variables, with reference to profitability of projected future exploitation of lignite deposit. The method presented in this article can be used best by variant selection for satellite or other perspective deposits.