All CDH components, including, but not limited to Hive, Impala, Kafka, Sentry, HDFS, Solr, Flume, as well as Cloudera Manager are free software. You can download and install on your cluster free of charge.
If you decide to do self support, i.e., resolve issues yourselves when you encounter problems, then you can use those software for as long as you want, and on as big the cluster as you like.
Cloudera only charges for subscription fees, which means our dedicated Support Engineers will help you to resolve any issues in your cluster as well as providing bug fixes and new feature you would require. Those subscriptions are node based as you can see from the pricing page.
So in short, CDK is free, we only charge if you need further support from us to help you resolve issues.