You get some number of days of free trial for it.
There are a number of free MP3 encoders out there, such as Lame, but I just coughed up the $20 and bought the Cakewalk one.
If I recall correctly, the Cakewalk one is the same as one as the free ones, but they are paying that money to whoever developed it as a license fee.
Anyways, here are some threads from a quick search I just ran:
http://forum.cakewalk.com/TUTORIAL-Getting-LAME-MP3-Encoder-to-Work-With-SONAR-m1255631.aspx http://forum.cakewalk.com/How-to-install-lame-mp3-encoder-m1990106.aspx http://forum.cakewalk.com/LAME-MP3-Encoder-m1568355.aspx http://forum.cakewalk.com/LAME-Encoder-lives-up-to-its-Name-m627968.aspx The above are from all sorts of versions of Sonar, but the principals remain the same.
Again, I skipped all of that by just forking out the $20 for the licensed one through Cakewalk, and have used it for multiple versions of Sonar, over the years.
I am very happy with it, but it's certainly your choice.
Hope the above helps,
Bob Bone