1. A non-transitory tangible computer readable recording medium comprising stored instructions that when executed by a computer, cause the computer to: identify, in a pricing formula, a price calculation operation to calculate a price from a price input variable, a mathematical operation to calculate successive values of a changing value variable, and a logical operation to conditionally perform an operation depending on a state of a logical operation input variable; select nodes from a node library of pre-stored nodes stored in the computer that are designed to be copied and assembled into a tree structure, at least one of the pre-stored nodes causing execution of a calculating operation by an electronic costing system to establish a price of a service, the selected nodes including a converter node, a processing node, an ending node, and a decision node; configure the converter node to store as parameters an initial input variable, the price input variable, and a unit conversion operation to convert the initial input variable to a unit of measurement compatible with the price input variable; configure the processing node to store as parameters the changing value variable, changed values of the changing value variables, and the mathematical operation; configure the decision node to store as parameters the logical function input variable and the logical operation; configure the ending node to store as parameters the price input variable and the price calculation operation; link each of the configured converter, processing, decision, and ending nodes to one of the other nodes by arcs; create a tree structure from the arcs connecting the selected nodes to one another, the arcs defining, by an ordered relationship establishing a sequence in which an operation stored in a parameter of a respective node is executed, an order in which said calculating operations are carried out by the electronic costing system, wherein the price calculation operation is executed last in the sequence to generate a net price, said tree structure being formed by i) the selected converter, processing, and decision nodes including a root node, father nodes, and child nodes and ii) the selected ending node extending from a respective one of the child nodes, wherein the price calculation operation is executed last to generate a net price; display on a screen, in a graphic form, said tree structure; convert the created tree structure, displayed in a graphic form, into a format directly readable by the electronic costing system; and store the converted tree structure in an information storing medium, wherein, the root node forms a root of the tree structure, each father node is connected to at least one child node by one of the arcs, each child node is connected from one of the father nodes by one of the arcs, each ending node forms an end of a branch of the tree structure, each of the father nodes and the child nodes being associated with a corresponding one of said calculating operations designed to be executed by the costing system to establish the price of the service, the calculation operations being executed based on the order defined by the arcs, and each ending node being associated with a corresponding operation for i) calculating a price and with ii) stopping the costing system from passing further through the tree structure.