DigitalOcean charges by the hour for all Droplets. The cost per hour is dependent on the hardware tier you select when creating a droplet and whether you decide to add the backup option enabled. Also please note that at minimum $0.01 is charged even if you use the lowest tier for even a minute as that’s the lowest that can be billed for. However, DigitalOcean sets a cap on expenses for each tier.
For example, if you had a droplet with the lowest tier, it charges $0.007 per hour. However, the cap for that tier is $5 a month. You can not be charged more than that for that single droplet.
So if you had a $5 Droplet and you’ve used your old Droplet for 10 hours and then destroy it, you will be charged only for those 10 hours: 10 hours * $0.007 per hour = total of $0.07.
As you are not paying up front for the whole month, you will not get a refund, but instead, you will pay for only what you’ve used.
Hope that this helps.