It is important to note that the power state of your droplet (on or off) has no effect on billing as resources remain reserved even for powered off droplets. To save a droplet and halt billing you can create a snapshot of it from the control panel and destroy it.
If you create a $10/mo droplet without the backup service enabled you will not be charged more than $10 for each calendar month for the droplet.
If you enable backups on your droplet this service adds an additional 20% to the cost of the droplet each month.
We are not currently charging for bandwidth used in excess of your plan limits but we will be in the future. An announcement will be sent out before this change occurs. Once this happens, if you were to use more than 2TB of transfer during the month, bandwidth over this limit would be billed at a rate of $0.02/Gb