Mobile data services grow rapidly in few years mainly due to prevalence of smart handheld devices. Under conventional pricing schemes such as flat, volume, and tiered pricing, network congestion occurs frequently in particular time intervals and locations. Meanwhile, WiFi networks have abundant capacity, but small coverage areas compared to cellular networks. In this paper, we study congestion-dependent pricing in heterogeneous wireless access networks and compare it with conventional pricing schemes. In congestion-dependent pricing, more mobile data traffic can be offloaded through WiFi networks by giving users incentives to deviate. A simple numerical result on temporally variable demand verifies that total revenue is increased about 13% in congestion-dependent pricing.