Cost Explorer pages helps you find out current and previous months' cost. You can open Cost Explorer in left menu under Account > Cost Explorer. Here an example report:
You can select the month to see the cost breakdown for that month. Explanation of the fields in the report:
Request: Total number of the requests sent to the database.
Storage: Average size of the total storage consumed. Lambda Store database has a persistence layer for durability of your data. If you have 1 GB data in your database through the all month, this value will be 1GB. If your database is empty in the first 29 days of the month and it expands to 30GB on the last day of the month; then still this value will be 1GB.
Cost: Total cost of your database is US Dollars.
The values for the current month is updated hourly, so values can be stale up to 1 hour.