If you use Confluent Kafka, the billing is pretty high. About 4 years ago it was much cheaper, but then they completely revamped the pricing to something ridiculous. I found that switching to Google Pub/Sub, at least if it meets your needs, is cheaper.
Kafka doesn't guarantee exactly once delivery at all, unless you're using Kafka Streams and even then your final output topic still won't get exactly once, the consumer group protocol doesn't allow for it.
It's cheaper until you get to a sizeable workload, and the P90+ latency is ridiculous.. the kafka api is weak and when you're not using kafka api you're limited on integration tools unless you want to be super locked in to GCP.
The lock-in argument is a non starter with me. I never see people move between clouds (well it happens but it is incredibly rare) and it isn’t because of lock in but rather they are pretty close to equivalent. And if you wanted to go on prem you can replace the messaging system, it isn’t one of the hardest steps of going on prem.
