With the recent popularity of mobile data devices, the demand for mobile data traffic has grown rapidly as never before. Hence, service providers are trying to come up with cost-effective solutions to battle this ever increasing demand for bandwidth in their cellular networks. Deployment of a denser heterogeneous network, with a large number of small cells, has been identified as an effective strategy not only to satisfy this unabated growth in mobile data traffic but also to facilitate ubiquitous wireless access. While the cost associated with on-site small cell equipment is low in comparison with a typical macro cell, deployment of small cells opens a new set of challenges, especially in relation to expenditures and capacity requirements associated with the backhaul. In this paper we discuss an efficient small cell backhauling strategy that leverages existing fiber resources in a cost-optimal manner. In particular, we formulate an optimization framework for planning a cost-minimized backhaul for a small cell network, which is based on the deployment of passive optical networks on top of the existing infrastructure. We demonstrate the effectiveness of our method by using our optimization framework to design a cost-optimal backhaul for a small portion of a realistic backhaul network. Our results show that in comparison to the typical point-to-point fiber backhauling approach, our technique can halve the costs associated with small cell backhaul deployment.