The problem under consideration is the cost estimation and consequent bid preparation for machined parts subcontracted to mechanical small and medium enterprises (SME). This activity, i.e. cost estimation and corresponding bid preparation, becomes more and more important due to the increasing and globalized competition in this market. There is, therefore, a clear need for precise and accurate cost estimation of machined parts in order for a small company to justify its prices. The proposed prototype software system is based (i) on a manufacturing feature based product description of the part to be machined and (ii) on a non-linear model of its process plan using Petri nets, taking into consideration processing alternatives and precedence constraints, which allows a heuristic based best search of the process plan and, consequently, the corresponding machining time and cost. Product description is done interactively through user friendly interfaces and the corresponding process planning model is constructed automatically in the form of a Petri net. Machine tools and their characteristics are selected from a customized database. Tools and machining parameters are selected through a link with the tool management software TOOL Light©. Minimum time or cost process plans and corresponding bids are reported using the Petri net model of the machined part under consideration and using machining heuristics. The type of parts that are considered in our application are rotational or prismatic parts that are used as components in complex machines like machine tools or automatic assembly machines.