In order to support bitrates beyond 400 Gb/s, DWDM networks are bound to evolve into a flexible-grid concept, where the spacing between optical channels is no longer fixed at the current 50 GHz. This shift opens up new possibilities in the range of transmission formats for optical channels of 40, 100 and 400 Gb/s. Using a static cost-based planning algorithm, we show that the capacity increase that arises from the use of multiple transmission formats within a flexible grid in metro/region networks is substantial. Also, we conclude that 400 Gb/s channels start becoming more attractive than aggregated 100 Gb/s channels for traffic demands over 300 Gb/s and a cost increase factor of 2,5 between 100 and 400 Gb/s.