Not sure if it's a tradmark violation or not - probably not since you could argue that it's a generic phrase. If you develop it with Harry Potter content they might come after you though.
It's hard to appraise since it is narrowly valuable to the author and publisher of Harry Potter books, and much less valuable to anyone else. Low $xxx for a fan site, maybe. You might get good PPC type-ins for a while, but most will go to the .com.