I don't think the values of a certain affix matter.
However I am very unsure if all affixes have an equal probability. If so, that (15/16)^60 formula for NOT getting arcane damage even once, is right. But if not, all these calculations are moot.
I only have some anecdotal evidence, but when I went to enchant some rares to have -lvl req as a secondary stat, I'd swear, I've seen experience per kill about 10-20 times as often as the desired stat. Please don't take this as proof for anything though, I'm just curious if there isn't some kind of weighting system in place.