Unless you're running a farm of these, the power cost differences is going to be largely unnoticeable. Like even in a country with very expensive power, you're talking a ~$0.10/USD per hour premium to have a 3090 at full bore. And that's assuming the M1 Ultra manages to achieve the same performance as the 3090, which is going to be extremely workload dependent going off of the existing M1 GPU results.