It's not cheaper for you, it's cheaper for them.

I am not nearly as knowledgeable as many of the commenters here on the hardware, but I do have first hand experiences running ARM on production at scale.

What AWS offers and what Google offers are not even close: on AWS, our workloads actually perform better (as in, higher QPS/core, resulting in fewer pods/instances), in addition to being more cost effective (both in terms of per-instance price to begging with, and less instances in total, due to aforementioned higher QPS/core serving same amount of total requests). For the limited ARM offer from Google we tested, the performance is worse than equivalent x86 instances, in addition to no price incentives.

On AWS it’s like a 20-30% discount - effectively free money for services like RDS, ElastiCache, and often Lambda or even ECS where the effort is either minimal or extremely low.