Source code under the GNU GPL or binary versions that allow you to create fonts under the SIL Open Font license.
5 USD or more
A version suitable for commercial fonts with upgrades for one year.
The latest version is birdfont-2.20.1.tar.xz. Source code for old versions can be downloaded from the release directory.
A flatpak is available for Linux x86_64.
A package is available for OpenBSD in graphics/birdfont.