In this paper, we assess a new technology, WDM Slotted Add/Drop Multiplexer (WSADM) which has been proposed as a new transport network technology based on subwavelength switching. This solution is envisaged to be deployed in the metropolitan area in order to improve bandwidth utilisation and minimise the overall cost of the network. In order to assess the relative cost of WSADM, we compare it, on the one hand, with OTN over WDM as a dominant transport solution in the current metropolitan network, and, on the other hand, with POADM (Packet Optical Add/Drop Multiplexer) and TWIN (Time-domain Wavelength Interleaved Network) as two other promising subwavelength switching solutions. The cost assessment is carried out over a ten-year period of time, taking into account the predicted traffic evolution. We consider a hub-and-spoke virtual network mapped on a physical topology inspired from a European operator network. The comparative study shows that, considering the adopted cost model, WSADM presents a lower cost than other technologies, and a limited growth during the studied period.