My Ruby on Rails application offers users to write short text messages and upload images.
I set a daily upload limit of 7 images per user and I created a background job to delete once a month all images older than 30 days.
I also set a limit of 300 users for my application.

Even considering that an image rarely exceeds 1 MB of size (and often is far from that), I would anyway take 1 MB as the average image size, if you think this is a sensible approximation.

• 7 images per user it is 7x300 = 2100 daily images uploaded to AWS from all users.
  
• 2100 daily images mean 2100x30 = 63000 monthly images uploaded to AWS
  
• 63000 images per month of 1 MB each require about 62 GB. I would take 100 GB as a safety value.

I inserted in the simple monthly calculator the following values in the standard storage section for US-East (Virginia):

• Storage: 100 GB
  
• PUT/COPY/POST/LIST Requests: 70000 (instead of 63000)
  
• GET and Other Requests: 2000000 (instead of 63000x30=1890000)

NOTE: 1890000 as far as I understand can express daily requests considering all users using the application once a day per 30 days, and thus every day asking AWS to show their images. Is that correct?

What I get is an estimated monthly bill of 3.31 $: is this a sensible estimate?
If instead my estimate is wrong, because I missed some point, considering that my monthly budget can be between 5 $ and 10 $, how would you suggest me to configure my application (number of users, number of images per day and their size limit)?