I'm curious if you have any posted benchmarks about it. In my own experience at work, m8a demolishes m8g core-for-core. The ARM64 core is much slower. I wonder if you're comparing vCPU-for-vCPU against Intel families that use SMT: that's one ARM64 core against one Intel HyperThread. Make sure you're using AMD.
In different regions the price difference is different. In us-east there is a 20% difference, in ap-south it is 50%. You can check for fargate ecs pricing for example.
e.g. https://www.phoronix.com/review/aws-m8a-m8g-m8i-benchmarks -- m8a won virtually all of these comparisons by a wide margin. Raw performance isn't really Graviton's selling point; price-performance is.