I managed to bring down ani.social's monthly costs to only ~14 USD when converted (which includes everything except backups). With 165 monthly users, that comes to around ~0.08 USD with a lot of accomodation.
Lemmy is efficient in resources except in storage (database and images) which grows infinitely. Unless you're purging older posts and images, it keeps growing (very slowly).
Right now: 0.3GB per user per month. This number is probably much higher for other instances because I don't keep copies of federated images anymore and I've been compressing images early on.
That's pretty high... Leads me to consider hosting an instance on local hardware. I know that's a whole security mess to deal with, but it night be worthwhile since I have lots of spare storage.