Expensive wood doesn't necessarily make it sound better. It usually just means it's rare, endangered, or has to be shipped halfway around the world. If you want to go exotic, go for it, by all means, but don't buy expensive wood thinking it's necessarily going to give you better tone.
There are exotic variations of a lot of "common" woods. Ash is available in figured varieties, and there's a stunning Japanese variety called tamo that's quite incredible to look at... dunno how it sounds, but if it's in the same family, it's probably close to some kind of Ash sound.
Rosewood is difficult to work because it's so dense. It's a very, very hard wood, and also very heavy.
Another wood i think would be good for downtuning is black walnut. You see it used on a lot of exotic basses, but it isn't as popular on guitars for some reason. There are also some beautiful burled or flamed walnut varieties. (such as Claro walnut, which is native to southern California)