There's been no details of the game released yet so I'll just offer my opinion.
I would say yes, that the game would probably be worth waiting for. Reason being it'll most likely be the last GNT game released for the Cube (not only because the Cube is at the end of it's life, but the story at this point has taken a dramatic turn and the whole game would need reworking to be accurate again), and you may as well hold out for the latest (and greatest?) one. Going off the changes between 2 and 3, the differences in the game engine will probably be minor at best. A few new characters, maybe a new mode, some new specials for a few characters and a couple tweaks is about what I'd expect. The turnaround time on all of the games has been about a year, so it should be due out sometime beyond October.
From my shopping experience, prices almost never drop in Japan. Unless it explicitly says "priced down re-issue" or something like that. And even that can take several years. Games usually just go out of print instead of being priced down (but I hear stuff is fairly easy to find in used shops).