This paper introduces a method for designing vertical ground heat exchangers and heat pump systems, by minimizing the total cost of the project. The total cost includes an initial cost composed of drilling, excavation, heat pump and piping network. An operational cost is also included to account for the energy consumed for heating/cooling a building. The procedure allows determining the optimal number of boreholes, their depth and spacing, and the optimal size of the heat pump. The method is tested for different ground conductivity and heat demands. The method can also be used to determine the economical viability of a TRT. For tested cases, results show that the excess cost due to uncertainty on ground thermal conductivity increases with the number of boreholes. Also, a cost sensibility analysis shows that the most influential parameters are the number of boreholes and their depth.