Cellular resources are expensive and should be saved whenever possible. In this paper, we address the problem of content dissemination in a capacity-constrained cellular network, a typical problem that future generations of networks will have to face. We investigate the benefits of device-to-device (D2D) communications with regard to resource savings in cellular networks and evaluate the cost of involving mobile users as relay nodes in the dissemination process. Thanks to their displacements, mobile nodes that already have a copy of the content opportunistically send it to neighboring nodes whenever they meet each other. To motivate the participation of mobile users, we consider a financial model where relay nodes are rewarded with a financial incentive. To conduct our study, we compare various relay node selection strategies including a random strategy and an Oracle strategy. Our results, obtained with a dataset of road traffic, show that the reward needed to secure the ratio of mobile users willing to act as relay nodes may exceed the cost of relying only on cellular transmissions. We evaluate the breaking point where this ratio comes with a higher cost for various network parameters including the range of D2D transmissions and the number of relay nodes.