Snapshots are charged at a rate of $0.05/GB per month. Pricing is based on the size of the snapshot, not the size of the filesystem being saved.
There is no additional charge for making a snapshot available in multiple regions.
Snapshots are available for all Droplets and Block Storage volumes across all regions.
Snapshots are independent from the Droplets used to create them. If you delete a Droplet without removing a snapshot based on it, that snapshot will still be accessible on the Images page in the Snapshots tab, and you will still be billed for it.