What is and is not billed
Runner time. Billed per second while a job executes. Provisioning before the first step and cleanup after the job are not billed.
Skipped work. A job that is not scheduled holds no machine. A reused step restores its recorded output. An unselected test never starts. None of these use runner time.
Cache hits. Reading from the cache is not billed. Cache storage is billed separately, at the illustrative per GB-month rate.
Verification. Reading a record, verifying a signature and exporting a receipt are included in the run.
The amount you save depends on your commit history. A change to a lockfile can select every test, and the bill reflects the runner time that actually ran.