I think those numbers are a good trick of marketing from AWS. I've been using Lambda for a couple of years now in my work and for my personal dev project but when I see on billing dashboard it cost certain amount of money even though my applications never reach thousand requests.
If you closely look at logs, a simple hello world function consumes certain MB of memory. In my case it was 48MB.
For me, this whole serverless thing is a marketing campaign from AWS. I know there are lots of benefits of Serverless but its is over hived from the giant.
I am not against the Serverless things. It is much more easy to get to market with Serverless.