Serverless instances offer operations-based pricing, meaning that you only pay for the Processing Units consumed by your database operations and storage consumed by your data and indexes. All operations are billed per day, and charged as follows (click image to enlarge):

In regards to read operations to the database, you are charged one RPU for each document read (up to 4KB) or for each index read (up to 256 bytes). Serverless instances may be more cost effective for applications with low or intermittent traffic. To learn more about pricing or see examples of how tiering is applied, view this documentation. You can also check out this blog, How to Optimize Your Serverless Instance Bill with Indexing, for tips on optimizing for lower costs.