Pricing of RDS Snapshot or backups

Question

Dear one,

I have confused to calculate pricing for AWS MYSQL RDS backups. AWS mentioned that "there is no additional charge for backup storage, up to 100% of total database storage for a region". But i couldn't get this point. Can any one clear my doubts? I will give one example. Kindly give some idea and how calculate for below case.

Example:

Cloud - AWS

Database - RDS

Region - Any one

Instance type - db.t2.medium

Storage size - 20 GB

RDS backups - Two additional backups per day (Backups retained for one month) and Monthly full backup (Backups retained for one year)

How we can calculate above things? I cant able to calculate this using aws calculator. I need to give some exact pricing for RDS backups. Kindly give me example calculation for above to understand. I hope you people understand this requirement. Its urgent too.