I completely read the AWS page for AWS WAF pricing, however I am still not sure how much would it cost if I create a single AWS WAF and hire just the AWS Managed Rule Set "Core Rule Set"
As I understand it should be 5 US/month for the WAF and just 1 US/mont for the complete Managed Rule Set because its rules have not been created by me and each Managed Rule Set costs 1 US/month, and that should be that
There is the possibility that the "Core Rule Set" would cost 23 US/month as it has 22 rules, and I am not completely sure that I will not be charged by them as I do not find it to be explicit enough from the Pricing page
So would it cost 6 US/month or 28 US/month? I find it quite a big difference bucko
You are correct with your first line of thinking. Each WAF is billed at $5 USD per month and each rule that you create is billed at $1 USD per month.
Note that rule groups that you get from the marketplace can be charged at any usage rate that the seller desires, but the flat pricing is always the same.
Basically the distinction here is rules created by you vs rules created by a seller or AWS. Rules that you create are always charged at $1 USD per month. Rules created by sellers or AWS are never charged to you directly
For your situation, you're looking at the Core Rule Set, which is a free managed rule group provided by AWS. You will be charged $1 USD per month for adding a rule set, but there are no additional usage charges for the free rule sets. This is in contrast to something like the Bot Control rule group, which is not free in that it charges based on usage on top of the $1 USD per month for any rule group.
I plugged these into the calculator at the bottom of the pricing page and got these results.
https://aws.amazon.com/waf/pricing