Footnote: In peer-reviewed literature, savings of 5% to 30% in annual energy consumption for lighting, cooling and heating due to optimized building and HVAC design has been reported.

To obtain an estimate of the return on investment for an office building with 10,000m2 floor area, assume that the average cost savings due to optimized building and HVAC design are 15%, that the average energy cost is $0.10 per kWh, and that the annual energy consumption is 200kWh=(m2 a).

Then, the savings due to optimized building and HVAC design are $30,000 per year.
Obviously, with the average price of a kWh rising annually the savings today is even more significant.

-Wetter, M. and Wright, J. (2003b). A comparison of deterministic and probabilistic optimization algorithms for nonsmooth simulation-based optimization.