I'll simply add here: I've switched to the Blaze plan during development, and while hitting 10's of thousands of reads, thousands of writes, thousands of Cloud Function Calls, etc my costs are generally $0.03 or less. The one exception to watch for is the [region].artifacts.[project-name].
appspot.com bucket, which can get large with Cloud Function build images - if you are deploying Cloud Functions repeatedly while bug-fixing. Even then it has to get to numerous GB before incurring charges like $1.30. Don't go insane with storage or calls (i.e. watch for infinite loops, etc), and the charges are minimal, and removes most of the limits.
LeadDreamer (Tracy Hall)