For multi-access networks with heterogeneous radio access techniques, the challenges include the problem of how to optimally distribute network traffics into each radio link in order to enhance the aggregated link capacity. As an effective load balance mechanism, traffics may be equivalently assigned to each RA, in proportion to its available capacity. To this end, airtime cost is identified as a common resource measure for WiMAX and WiFi links. Due to their different data transmission mechanisms, the calculation of airtime cost is separately derived. Based on the common metric, the offered traffic for each link is converted into airtime cost required for its transmission. Traffics are distributed so that cumulative airtime cost between RAs is fair. Evaluation results show that airtime cost model can commonly measure the link resources of heterogeneous wireless links, and this measurement enables traffics to be equally distributed, in proportion to their transmission capacity.