All serverless instances charges are based on actual usage (https://docs.atlas.mongodb.com/billing/serverless-instance-costs/) meaning you only have to pay for operations you have performed and storage consumed by your data and indexes.
If you do nothing at all (i.e. no queries or writes) and store nothing at all (i.e. no documents stored on your serverless instance), then at the end of the month, you will be charged $0.
Storage being billed prorated per day, means if you’ve stored 1GB of data for 15 days, then you will be charged approximately $0.125 ($0.25/30*15).
Hope that helps.