Average ~$3500/mo, but that can go up and down depending on organic traffic, competition (who's running ads), and world events (2-3x'd for a month or two during peak Covid lockdown). It was running ~$1-1.5k for a good chunk of the time when I was building out my own newsletter system, and I wasn't super eager to throw 30-50% of my revenue at something off-the-shelf.