The overarching principle of automating email subscriber growth: trade value for value.
Do this by creating an optin funnel (or survey funnel, lead magnet funnel, squeeze page funnel, etc.) to allow a potential subscriber to exchange their email address for something of value (guide, video series, training, etc.).
[See an example here.]
Send traffic to the optin funnel.
Immediately sell things on the back-end of this optin funnel to fund your advertising expenses.
Learn more about automating your email subscriber growth by clicking here.