Well, take care of the school stuff first, as that will be with you forever. The boxing is fun, but its definitely secondary.
Part of the gem thing, is the cyclical nature. It goes up with new content or a new arena season, and then gradually falls as most people get their upgrades. With the release of Cataclysm a ton of people will be gearing up again, so the prices should rise. Not sure if there will be epic gems immediately, or if the gold will be made on elemental transmutes such as Earth to Water or what not.
500k in gearing toons and other expenses is quite a bit, but as long as you have your nest egg for the start of Cataclysm, you'll be able to level the alchemy fairly fast, which will let you experience the high selling points of the expansion... release, 4.1, 4.2, and 4.3 along with their respective arena seasons.
The DK and Ret team, its got a lot going for it.
And from what we've heard, it will retain most of the good features in Cataclysm too.
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