1. A computer-implemented method for processing rules in an inference system in order to determine a value of a target variable by an inference engine, the method comprising: determining the value of the target variable on the basis of a reasoning sequence comprising a set of rules to define conclusions depending on premises, the premises being functions of values of variables, wherein the variables are one of: source variables, each having a source variable cost that defines a cost of acquisition of a value of the source variable, the source variable cost being stored in a database; and, regulated variables, each having a value dependent on the conclusions of the rules and a regulated variable cost that defines a cost of acquisition of the value of the regulated variable, the regulated variable cost being dependent on the cost of each variable in the premises and the conclusion of a particular rule, controlling an operation of the inference engine by an inference control module; and determining, by the inference control module, the reasoning sequence by: reading the source variable cost of each source variable from the database; identifying target rules that allow to determine the value of the target variable; determining the cost of evaluation of premises and conclusions for each target rule based on the source variable costs and the regulated variable costs for variables associated with that target rule; evaluating the least costly premise in the least costly rule and recalculating the cost of evaluation of premises and conclusions for the remaining rules, based on results of the evaluation; and, returning to evaluation of the least costly target rule until the value of the target variable has been determined.