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.)

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.