Mobile data offloading is an emerging technology to alleviate cellular network congestion and improve user service quality. In this paper, we investigate the economics of mobile data offloading through access points (APs) deployed by small cell service providers (SSPs), implementing uniform volume prices for all the mobile users (MUs) in each SSP's coverage including the overlapping area. In particular, we consider a data offloading game with a single mobile network operator (MNO) and two SSPs with overlapping coverage areas, where each SSP announces a uniform price for offloading the cellular traffic within its coverage, and the MNO determines the traffic volumes to offload. We show that there is no pure Nash equilibrium (PNE) in the data offloading game, and prove the existence of the mixed strategy Nash equilibrium (MNE) by price randomization. Furthermore, we propose a simple one shot auction mechanism that is easy to implement and has PNEs which yield SSPs the same equilibrium payoffs compared to the expected payoffs obtained at the above MNE.