It's just a landing page you are looking at for which I wanted to put down some numbers. The product [1] is very much pay-per-use (per e-mail sent), so the number shown here is more of a minimum monthly charge.
That said, I will probably double that number (and the included usage) prior to launching.
I expect there is a large difference in the quality of your users whether it's actually free or it's $4 per month.