Some embodiments of the invention provide a method of costing routes for a set of nets. The method identifies at least one route for each net, where each route has a particular length. It also identifies an estimated route length for each net. It then computes a cost that includes an exponential expression for each net. Each net's exponential expression includes a base and an exponent. The exponent of each net's exponential expression includes the length of the net's route divided by the estimated route length for the net.