It is definitely a high-spend area. It is not uncommon for teams that are <50 people to spend ~$2-5000/month on CI, sometimes more.
There are repos we've seen that are heavily sharded for testing and trigger ~50-200 separate jobs per commit, each taking a few minutes to run.
CI costs are only going to increase with each engineer putting out 10x code and autogenerated tests in the next few years.
aside: we're tackling this problem as a company. I've been considering putting out an analysis of this data from our customer set - your post might just trigger me to do it.