Condition-based maintenance critically relies on efficient and reliable structural health monitoring systems, where the number, position and type of sensors are determined according to rational and principled criteria. This paper proposes the use of the value of information and the relative expected information gain as optimality criteria to determine the best number and positions of sensors, respectively. The proposed methodology is general, but in this paper it is specialized for ultrasonic guided-wave optimal system configuration. Two case studies are used to illustrate the suitability of the proposed methodology in providing the optimal sensor configuration of an ultrasonic guided-wave based structural health monitoring system. The results confirm the value of information as an efficient and rational index to compare among different sensor positioning strategies, while accounting for the underlying modeling and measurement uncertainties. As key contribution, a novel framework that trades-off between amount and cost of information is provided. The results show that geometrically unconstrained sensor configurations are preferred, since they provide a healthier balance between the amount of information and the benefit of such information.