https://aws.amazon.com/msk/pricing/ prices scale linearly with CPU beginning with m5.large, and I wouldn't really want to run a production Kafka on anything less than m5.xlarge. (They do at least keep linearly scaling all the way up.) Speculating wildly, I could probably have run some of our real clusters on the equivalent of a 8xlarge, but of course 32 core systems were not widely available at that time. The cluster I run today, even a hypothetical 48xlarge would struggle.

YMMV for non-managed stuff, but really, you can only bump cores like 3 times realistically, 4 if you started really shitty, before you start getting into special pricing brackets.

Increasing core count is not really vertical scaling. It's a hybrid between vertical and horizontal scaling, having some characteristics of both. It also tops out quite early (especially its cost-effectiveness for many use cases, but there's an absolute upper limit as well).