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The problem under consideration is the cost estimation of operation sequencing for non-linear process planning, i.e. taking into account processing alternatives. In order to determine overall costs for feasible process plans we take into account in our Petri net model costs caused by machine, setup and tool changing in addition to the single operation cost. We introduce a new Petri net model that allows the application of cost analysis algorithms. This is a PP-net (Process Planning net) which represents manufacturing knowledge in the form of precedence constraints and incorporates machining cost, machine, setup and tool information in each transition. We show that the PP-net allows the calculation of the optimum process plan without the need to first develop all possible solutions. We apply the developed methods and calculate the optimum process plan to an industrial case study of a mechanical workpiece of moderate complexity.