There is a free version, its called unrar and preinstalled on many Linux Distros. You can only uncompress, which is 200% enough for that stupid format XD
According to Wikipedia there are two versions of unrar, a free one which supports older versions of RAR, and the proprietary RARLab one that supports newer versions.
There's also a tool called The Unarchiver that supports RARv5 although it has been bought out and is currently proprietary, the free version is still available. Theoretically it could be used to provide RAR decompression for apps such as this one.