We present a large scale empirical evaluation of Optical Transport Network (OTN) as a cost optimization strategy for IP transport infrastructures. Using operational and cost data from 12 operators, 45 transport platforms, and 12 representative network scenarios, we compare OTN with legacy SDH/DWDM across metro, regional, long haul, and international backbones. A hypothesis driven framework combining bottom up cost modeling, paired t tests/ANOVA, regression, and Monte Carlo sensitivity analysis quantifies impacts on CAPEX, OPEX, energy, bandwidth efficiency, and scale economics. Results show statistically significant reductions of 35–40% in CAPEX and 33–42% in OPEX (p < 0.001), driven by wavelength reduction via sub wavelength grooming, improved bandwidth utilization (~40%), equipment consolidation, and superior energy efficiency. Power per transported gigabit falls by 46–52%, accounting for nearly half of OPEX savings. Economies of scale are evident: cost advantages rise from ~30% at 100?Gbps to >40% in 10–40?Tbps regimes. A five year Total Cost of Ownership (TCO) analysis indicates 36–39% overall cost reduction, with 8–14 months break even even in brownfield migrations. These findings provide robust evidence of OTN’s economic and sustainability advantages and offer actionable decision support for operators, vendors, and policy makers pursuing cost efficient, scalable transport modernization.