Can anyone recommend mailing list hosting for a non-public group of about 20-30 people that costs less than $100 per year?

My family plans all of their reunions via e-mail and every year we end up with someone putting an out-of-date e-mail for someone in the list, or forgetting to include someone, or including someone who has asked to not get updates. I offered to host a mailing server, and I may just end up rolling my own on a DO droplet with mailgun or something for reliable delivery because mailing list hosting I could find was rather expensive.

Gaggle Mail (https://gaggle.email) has a free plan and it's cheapest paid plan is $100 per year. It's popular with family groups doing what you describe.