Databricks starts to charge for DBUs once the virtual machine is up and the Spark context is initialized, which may include a portion of start up costs, but not all. Init scripts are loaded before the Spark context is initialized, which therefore would not be included in DBU usage.

More info here: https://forums.databricks.com/questions/15245/are-dbus-assessed-for-when-instances-are-booting-u.htm...

Cluster Best practices for cost optimization: https://docs.databricks.com/administration-guide/capacity-planning/cmbp.html