How to estimate correctly cost on AWS Backup ?

Hello All, I'm currently working on AWS Backup service and I'm trying to estimate the cost of the service. I tried to use aws calulator but I think the price is not correct cause it doesn't count the growth rate of my data. For my example I have :

• 10 GB of data on DynamoDB
• Everyday 5GB are created ( so I will have 15GB next day )
• Backup frequency is 1 backup every week
• Retention data is 6 days
• so 150 GB are created every month

In my calculation : I calculate the cost of saving 1GB / day = 0.0036$ GB/day I calculate the size of the data for each backup. 1st = 45 GB => 0.97 $ = 45GB * 0.0036$ GB/day * 6 days 2nd = 80 GB => 1.72 $ 3rd = 115 GB => 2.48 $ 4th = 150 GB => 3.24 $ So the bill will be 8.41$

And on aws calculator I got this result : Daily Retention (GB): 0 6 days warm retention (weekly) / 30 days per month = 0.20 Monthly retention (weekly) 0.20 Monthly retention (weekly) x 150 Total storage = 30 Weekly Retention (GB) Weekly Retention (GB): 30 Monthly Retention (GB): 0 150 Total storage + 30 weekly Retention = 180.00 Warm Capacity (GB) 180.00 warm capacity x 0.11 USD backup price = 19.80 USD average monthly warm USD Average Monthly Warm: 19.80 USD Total DynamoDB Backup Cost (monthly): 19.80 USD

But i don't understand how aws calculator got 19.80$. Why does it add 150 total storage in calculation? As you can see, I'm little bit lost

Thanks for your help apoon