Not the case, Aaron. If you own the version supplied with PA9, you can use it in Sonar. You do not need to buy it again.
I forget the exact procedure, but from your Sonar CD look for the .mp3 encoder install program. Run it, and when you get to a certain point it will ask where you got the encoder. PA9 should be one of the choices. After that it will ask you to pop in your PA9 CD to verify.
After that, you should be good to go, with the 30-day limit removed.
In all honesty, though, save yourself the trouble and get the Lame encoder. I have PA9 version and have never even thought about bothering to reinstall it.