Phish concerts are a hot thing, but the true die hard Phish phans are ones that like to trade music. There are entire websites and newsgroups dedicated to trading Phish shows. I would consider myself a pretty big fan, but I never really got into the trading scene.
So speaking from your perspective, if your CD's are of store bought quality, you are charging too little. I got their 6 disk "Hampton Comes Alive" box set for $65, so I think you can do better than $20.
And when you think about it, Phish is more of a "live" group anyway. As a matter of fact, they don't make their money on CD's, they make it at their concerts. I heard from a friend of mine that in 1999 or thereabouts, they made $35 million in concerts alone. Pretty wacky if you ask me.