I don't this this is explained anywhere, and I haven't heard of anyone doing enough testing to figure it out yet (I certainly haven't). This is something I'd like to know too.
I believe your inclinations are correct: it would cost more Arcane Power to use a higher attack per second wand.
Here's how I suspect it works:
- Faster attack speed makes you attack faster with all abilities
- Higher attack damage makes you hit harder with all abilities
- Two weapons with the same DPS but different attack speeds will cost different amounts of power (Mana/Hatred/Spirit/etc.). There is no adjustment for speed.
In addition, most characters (Monk, Demon Hunter, Barbarian) have power-generating abilities. Attacking faster with those abilities means faster power generation, which would offset the fact that attacking faster with power-spending abilities makes you spend power faster.
I don't have a good source for this, but I am inferring based on my experience, my thoughts on how I would do it if I were designing the game, and the Monk ability Fists of Thunder which specifically states that it is inherently faster and therefore generates more Spirit than other stuff.
The power-generating abilities aside, straight damage values might make it seem like stronger, slower weapons be strictly better because they cost less to put out the same damage as fast weapons. However, damage isn't everything. When you attack slower, it takes you longer to cast each spell. You can't react as quickly, and you might miss more often due to enemies moving around. It's harder to take a swing (or shoot some lightning, or launch a kick) and move afterward. Try out a Barbarian with some giant axe and you'll feel a definite difference.
I have a feeling that mobility is going to be important, especially in harder difficulties. This would mean that being able to poke quickly and change position, or get a bunch of power in a hurry, will be valuable. How valuable it is, though, you'll have to decide for yourself based on your own judgment and playstyle.