The game itself is free, but there is also hat DLC! I have it on good authority that it is a very good hat and playing Frog Fractions wearing it feels as though you are playing an entirely new Frog Fractions game.
You can buy the hat for $7.99 (if the launch discount is still happening as you read this) or along with Glittermitten Grove in the Frog Fractions Cinematic Universe bundle.
Also here's the press kit if you're into that sort of thing!