Billing is based on hourly rate. You pay for how much hours you used Droplet in month.

Example: Let’s say that you used Singapore ($10) Droplet for 50 hrs this month. That means if you destroy it now, you’ll be charged only $0.75 (50 hrs * $0.015 for $10 Droplet) for it. If you create new Droplet (e.g. in USA) and use it for whole month left, you’ll pay for how much hours left in month. So basically you’ll pay $10.

That same logic applies to Volumes and Snapshots too.

You can learn more about billing process in Billing FAQ.