Since the introduction of smartphones and tablets into the market, mobile data traffic has shown an exponential growth. To cope with such mobile data explosion, offloading techniques were proposed. The main aim of offloading is to avoid transporting unnecessary traffic in costly networks without creating any additional incremental revenue. Offloading is addressed in this paper where we focus on the the interworking between 3GPP access and WiFi. Thus, we propose an economical model for offloading, capturing simultaneously the subscribers' conditions and the operators' preferences. Such model enables the operators to specify an offload strategy to increase their economical benefits without impacting their subscribers QoE.