We target the minimum cost design of a 5G network exploiting Small Cells (SCs) carried by Unmanned Aerial Vehicles (UAVs). In our architecture, UAV-SCs can be recharged by a set of ground sites, which provide energy capabilities thanks to the exploitation of micro-generation (i.e., locally produced energy) and batteries. In addition, the ground sites are connected by means of a set of optical fibers forming a tree-based topology. We then define an optimization problem to minimize the total installation costs of the ground sites, the optical network, the batteries, and the sources of micro-generation, while ensuring the coverage of the territory through the UAV-SCs. Results demonstrate that it is possible to notably reduce the total costs compared to a legacy solution, which assumes the installation of fixed Base Stations (BSs) to provide full territory coverage.