Hm… I think you’re off by a factor of two there, actually. Fly.io quotes prices in USD, and their lowest tier (“Basic”) for managed databases is $38/month. You would need to pay for storage (an extra ~$1/month, for small fry) and the webapp’s separate resources on top of that.
You may have gotten a lower bill this time if you created the database in mid-October. In that case, they would only charge you for the remaining days in that invoice period.
Right, their current tiers are clearly designed for ongoing businesses with steady customers, etc. There’s a proposal already for a “Broke” tier that would be more affordable, and Fly.io has expressed sympathy for that. But they will finish rolling out the higher-margin ones first, I expect.
There’ve been some recommendations offered for alternative managed Postgres providers in forum threads, as a consequence, although I haven’t used these myself:
https://community.fly.io/t/managed-postgres-starter-plan/25884/6
https://community.fly.io/t/managed-postgres-starter-plan/25884/13