Azure eval account got disabled recently as I had exceeded the monthly limit. When I looked closer I had been charged 4 database units on a single day. I was bemused because only ever had 2 databases (Prod and Staging) so I raised a support case with Microsoft Azure team.
To cut a long story short , Microsoft have confirmed that each time you create a database you will be billed for 1 database / day unit , regardless of whether you are creating the same database each time !
In my case I drop / re-creating Staging db several times within a few minutes as I was testing out a release process that went wrong.
The support rep couldn’t show me a link that showed this as being documented. They have promised to follow this up.
You can query below from master db to see the usage per day as well as the looking at billing in the Azure portal
SELECT * FROM sys.database_usage
ORDER BY TIME DESC
Andy Ball's Blog 