Email is still one of the cheapest ad channels. The cost to reach a subscriber is equals to how many subscribers you have, and the plan you are paying.
Even better, the cost stays the same if you send 1 or 100 emails per month (not that you should send that many emails to decrease the cost per reach.)
There are a few things you can do to drop the cost even lower. In any contact list, you will have people that never opened an email from you. I call them “zombies.”
The output will look something like this:
Please note that the zombies can be actually zombie bots (that found a way to bypass your form captcha validation.)