They could do a totally killer storyline going into Mania with the main eventers they have NOW (Austin-Rock in a grudge match and HHH-Jericho for the title), but bringing in the nWo (Hogan, Hall and Nash) just totally confuses things, is going to make a mess of angles that they've started, and won't do anyhting for the company. Hall & Nash make sense, but Hogan is an old piece of crap who isn't worth the trouble.
These three reformed the nWo in 1999, and it killed the company. Why should we think it'll be handled right this time? I mean, sure it sparked a lot of interest in 1996, but why is there any reason to believe this will work again? Doesn't Vince remember he turned his company around by creating new ideas, NOT reliving old ones, and reviving the nWo isn't just beating a dead horse, it's taking the horse and trying to pretend he's healthy and ready to ride.
Eh, there's tens of millions of dollars in wrestling; I'm not overly surprised that a WrestleMania main-eventer would get $600K for his part in the event. (Especially Orton, who's one of the golden boys.)