In this study, the authors investigate a maximum incentive that a primary user (PU) can charge a secondary user (SU) on SU's power consumption devoted to delivering its own signal. As a constraint, the PU also should achieve its data rate at a predefined level. Working as a half-duplex relay, SU hears PU's signal in the first phase and sends its own signal superimposed on PU's signal in the second phase. SU optimises power allocation between the two signals in the second phase, accounting for the expense-benefit trade-off in sending its own signal. Depending on SU's receiver mode or the number of SU's receiving nodes, SU's decision on power allocation is differently optimised. The authors also provide optimal incentive for PU by optimal pricing for different occasions. The optimality is proved in this study. Since the optimality is purchased in practice when PU has perfect knowledge of the SU's response on the PU-issuing price, the authors also propose an efficient interactive price-searching protocol between distributed PU and SU. Numerical investigations are given to verify and illustrate the optimality of the proposed price as well as the protocol.