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We discuss the emerging rate-adaptable optical transmission technology and how this new technology may be employed to further reduce the transport network cost to meet ever growing bandwidth demand in the core network. Two different types of transponders are considered: those adjusting either the transported bit rate (i.e., client data rate) or the symbol rate (with a fixed bit rate). We propose a methodology for calculating the (normalized) cost to build out an entire long-haul transport network with several options for bit-rate-adaptable transponders. By using link-length demands from an exemplary distance-diverse network, we demonstrate that time-domain hybrid-QAM-enabled fine-grain rate-adaptable transponders can reduce network cost by more than 20 percent within a traditional, fixed-bandwidth, wavelength-division-multiplexed grid. We also argue that the total transponder expense using symbol-rate-adaptable technology will be greater than when using bit-rate-adaptable technology, as well as requiring more costly flex-grid ROADMs for channel routing.