I like Realm but I agree, $30 for 100 user-device that users log in from at least once a month is harsh, and discount volumes are unknown from just reading the pricing page.
I'd assume if you're trying to be ad revenue-based and "accidentally" have more than 30 users just by being in the store, then Realm Free tier is going to throw you out quite quickly. So that's more like personal usage if anything, and even then you need to manage your own backups.
Does MAU even mean registered user, or execution on different device? Does a user who registers twice count as two users?
What's worse is that if the dev edition suited your needs previously, then the new Realm Self-Hosted pricing is also an unknown. As you can't not have Server-Side Realm Sync Access anymore, does that make it $1750/month at minimum?
So many unanswered questions... although when I was in the chat, all I could think about is "I can't know if this is affordable", and otherwise had no questions at all.
(edit: aligned with MAU -> MAD)