The relative error of cloning of quantum states with arbitrary prior probabilities is considered. It is assumed that the ancilla may contain some a priori information about the input state to be cloned. The lower bound on the relative error for general cloning scenario is derived. Both the case of two-state set and case of multistate set are analyzed in detail. The treated figure of merit is compared with other optimality criteria. It is posed that the relative error is more expressive in the case of nearly indistinguishable states. The quantum circuit for optimal cloning of a pair of pure states is built.