I'm only a customer and this isn't an official response, but there is no charge for storage of backups or snapshots. I've been here since March 2012 and I've never gotten a charge for storage of backups or snapshots. Hope this helps.
from the control panel:
Please note: We will begin charging for automated backups starting on July 1st, 2013. You will receive your first bill for enabled backups on August 1st, 2013. Pricing is set at 20% of the Droplet's monthly cost (e.g. It will cost $1/mo. to enable backups for a 512MB Droplet).