How much cost transfer/bandwidth with CloudFlare CDN?

I can’t find any usefull information about bandwidth cost.

I found this hyperlink:

Ok, so in some cases there is no bandwidth costs.
But how I could calculate additional costs based on S3 or nginx serving static files?

If you have static content make sure to add a pagerule to cache everything with edge ttl to something like 1month. You will see significantly low bandwidth costs. Cached pages may never reach your origin server and it saves lots of bandwidth and load to your server.