In the course of sales process of Enterprise Resource Planning (ERP) Systems, it turns out that the standard system must be extended or changed (modified) according to specific customer’s requirements. Therefore, suppliers face the problem of determining the cost of additional works. Most methods of cost estimation bring satisfactory results only at the stage of pre-implementation analysis. However, suppliers need to know the estimated cost as early as at the stage of trade talks. During contract negotiations, they expect not only the information about the costs of works, but also about the risk of exceeding these costs or about the margin of safety. One method that gives more accurate results at the stage of trade talks is the method based on the ontology of implementation costs. This paper proposes modification of the method involving the use of fuzzy attributes, classes, instances and relations in the ontology. The result provides not only the information about the value of work, but also about the minimum and maximum expected cost, and the most likely range of costs. This solution allows suppliers to effectively negotiate the contract and increase the chances of successful completion of the project.
 Plecka P., Bzdyra K. - Wykorzystanie ontologii w wymiarowaniu projektów informatycznych [at] XVII Konferencja Innowacje w Zarządzaniu i Inżynierii Produkcji. Zakopane 2014.
 Parry D. - A fuzzy ontology for medical document retrieval [at] ACSW Frontiers '04 Proceedings of the second workshop on Australasian information security, Data Mining and Web Intelligence, and Software Internationalisation 2004, Vol. 32, pp. 121-126.
 Lee C.S., Jian Z.W., Huang L.K. - A fuzzy ontology and its application to news summarization [in] Systems, Man and Cybernetics. Part B: Cybernetics. IEEE Transactions, Vol. 35, No. 5, 2005, pp. 859 -880.
 R., Song D., Li Y., Cheung T., Hao J. X. - Toward a Fuzzy Domain Ontology Extraction Method for Adaptive e-Learning [in] IEEE Transactions on Knowledge & Data Engineering, Vol. 21, No 6, 2009, pp. 800-813.
 Cui G., Lu Q., Li W., Chen Y. - Automatic Acquisition of Attributes for Ontology Construction [in] Computer Processing of Oriental Languages. Language Technology for the Knowledge-based Economy, No 5459, 2009, pp. 248-259.
 Carlsson C., Brunellii M. Mezei J. - Decision making with a fuzzy ontology, Soft Computing [in] A Fusion of Foundations, Methodologies and Applications. Special Issue on Fuzzy Ontologies and Fuzzy Markup Language Applications. Springer- Verlag Berlin, Heidelberg 2012, Vol. 16, No. 7, pp. 1143-1152.
 Alexopoulos P., Wallace M., Kafentzisi K. Askounis D. - IKARUS-Onto: a methodology to develop fuzzy ontologies from crisp ones [in] Knowledge and Information Systems, Vol. 32, No 3, 2012, pp. 667-695.
 Czarnecki A., Orłowski C. - Ontology Engineering Aspects in the Intelligent Systems Development [in] Knowledge-Based and Intelligent Information and Engineering Systems, Springer Berlin Heidelberg 2010, pp. 533-542.
 Czarnecki A., Orłowski C. - Ontology as a tool for the IT management standards support [in] Agent and Multi-Agent Systems: Technologies and Applications. Springer, Berlin - Heidelberg 2010, pp. 330-339.
 Zadeh L.A. - Fuzzy sets [in] Information and Control, Vol. 8, No. 3, 1965, pp. 338-353.
 Van Leekwijck W., Kerre E.E. - Defuzzification: criteria and classification [in] Fuzzy Sets and Systems, Vol. 108, No. 1, 1999, pp. 159-178.