Ground source heat pumps (GSHPs) provide a clean heating solution, but their application in severe cold regions is limited by low energy efficiency, excessive borehole length, and soil thermal imbalance. Medium-deep geothermal systems offer a promising alternative by utilizing higher subsurface temperatures. However, research on coaxial borehole heat exchanger (CBHE) performance in severe cold regions remains limited, especially regarding techno-economic analysis for low-energy office buildings and comprehensive evaluation methods incorporating subsidies and comparisons with district heating. This study investigates a medium-deep CBHE system in Harbin, China, through field testing and economic analysis using both static and dynamic payback periods. Results show that the system maintained a stable indoor temperature of 21.72 °C when outdoor temperatures ranged from −25.1 °C to −4.2 °C. The maximum source-side COP and heat pump COP reached 6.89 and 6.14, respectively. Heat extraction increased by 8.34 W/m for every additional 100 m of borehole depth, indicating deeper wells are more cost-effective than adding more boreholes. The initial investment was 211 CNY/m2, and annual operating costs were 20.21 CNY/m2. Without subsidies, economic competitiveness is limited; however, with grid-connection subsidies, the dynamic payback period is 8.4 years, demonstrating favorable economic potential. These findings offer theoretical and practical guidance for optimizing geothermal heating systems in severe cold regions.