One way would be to calculate a cost per game, factoring in both electricity and an amortized cost of the hardware, maybe having a penalty too for extra time run (e.g., if focusing only on hardware depreciation and electricity, 1 minute of TPU would translate to 2 weeks of CPU, that 2 weeks of waiting still costs you something). Obviously this isn't stable, as relative prices of GPUs and memory shift over time, and it's somewhat sensitive to setup; but done right it's probably more "what a user actually wants to know", in terms of what it would take to get equivalent performance.