We offer a generous free tier which doesn't limit your number of projects, never pauses, and available for commercial use. The pro plan is $30/mo and then pay for usage.
We love the small API and fast getting started experience of Firebase. We take a lot of inspiration from them for our write api. Hand-rolling joins was often a pain point though and we thought a graphql-like interface was a better experience.
[1] https://www.instantdb.com/examples?#8-merge-tile-game
[2] https://github.com/jsventures/stroopwafel