I remember getting the Nocturne guide for $10 at EB a long time ago. Even though I end up selling the game on ebay, the guide was definitely worth it, even at asking price.
As for the Persona 3 guide, I'm not sure if it's worth $20. I've never played a Persona game (even though I have owned Persona 2 for a while) so I can't really tell. It doesn't use glossy paper like Disgaea 2. It does give recommendations on what to do day per day and seems well laid out. I'll know whether it's good or not once Persona 3 comes out. Lastly, I'm also not a guide person as well, so sorry if I offered nothing useful in comments.
Oh, and I got this by flipping at EB so technically it cost me nothing.