But how much this costs extra? Sounds like you are using cloud-provided k8s.
Negligible for me personally, it's much less than either our EC2 or RDS costs.
A job ago we had our own k8s cluster in our own DC, and it required a couple of teams to keep running and reasonably integrated with everything else in the rest of the company. It was probably cheaper overall than cloud given the compute capacity we had, but also probably not by much given the amount of people dedicated to it.
Even my 3-node k3s at home requires more attention than what you described.
The amount of work/cost of using k8s for handling them in comparison to doing it "old style" is probably negative by now.