JEM Vs. RG5XX
The Jem and the 5XX series both have the same trem, an Edge. Both are made in Japan. Jems are made by the same folks who make the J Customs and the Prestige guitars, so the build quality is a bit better.
Jems have better pick ups, which you may or may not want to swap anyway. RG 5XX's have crap pickups.
There you go. Is the Jem worth twice as much? Maybe not, but I'd probably still buy the Jem, because I'm a financially irresponsible idiot, that loves cool guitars more than a lofty bank account. Plus, shame on me, I value how cool a guitar looks as much as how good it plays.
Someday I'd like to get the DBK, because that paint job is cool, and the neck is awesome.