Dense Wavelength Division Multiplexing (DWDM) transport networks based on fixed spectrum grids with Single Line Rate (SLR) wavelengths have difficulty managing the exponential growth and heterogeneity of data traffic, and the rapidly expanding Data Center Interconnect (DCI) market. Alternatively, higher scalability can be achieved by adopting Mixed Line Rate (MLR) architecture and Elastic Optical Network (EON) architecture, which is enabled through the use of Optical Orthogonal Frequency Division Multiplexing (O-OFDM) technology. In this paper, we develop heuristics for the design and planning of optical transport networks using the aforementioned architecture options. We use these heuristics to compare the total cost of the innovative elastic network with respect to conventional DWDM networks (SLR and MLR) operating in different reference network models, and under different traffic patterns. Our simulation results provide equipment vendors and network operators economic-related insight regarding the cost benefits of the elastic OFDM-based paradigm for the operation of future-proof data center optical networking.