Have you ever wondered...
How many kilobytes in a Petabyte, Exabyte or Zettabyte?
The table on the right shows the math. Be aware that while some services are billed based on storage size, in many cases storage is less expensive than bandwidth.
When referring to back-ups the file sizes you should consider the number if iterations required to meet your needs. In other words you may want weekly backups saved for 1 month plus monthly backups saved for 3 and quarterly backups saved for 1 year. so the file size of the information on media and devices is larger than a single instance.
Long term you must also have a device capable of interpretting what you have on backups.