% Cast Speed to % Spell Damage value?

I'm assuming this has been calculated somewhere - but I can't seem to find it.

Does anyone know what the weighted value of % cast speed to % spell damage is?

i.e.
1% cast speed = 5% spell damage (or whatever elemental damage)

depends on your skill- just compare your dps after switching out similar wands

There's no such equivalence, because the relative value of one depends on the amount of the other you have.

A simplified damage formula can be written by reducing all other factors to a constant, and then redefining our units so that constant is one.

Let D be damage, S be spell damage, C be cast speed:

D = (1+S) * (1+C)

S in this case being:
S = spell damage + elemental damage + single element damage + projectile damage...
(or whatever of those modifiers applies for your attack)

The partial derivatives are then obviously:

dD/dS = 1+C
dD/dC = 1+S

What does that mean? If you have 100% spell damage and 0% cast speed, cast speed will be twice as good as spell damage, IE 1% cast speed = 2% spell damage in terms of damage.

More realistic numbers would be something like 300% spell damage and 50% cast speed. In this case:

dD/dS = 1.5
dD/dC = 4

So 1.5 cast speed is the same as 4 spell damage, in terms of your damage output.

TL;DR: Given these typical values, you could generate a "rule of thumb" that cast speed is about two and a half to three times as good as spell damage. Keeping in mind that it is only valid in a typical gear range.

I tend to avoid a lot if not most cast speed as you use more mana to get extra DPS, compared to just increasing it via raw damage and maintaining lower mana cost. If you can support the mana cost, cast speed is worth it.

Keep in mind it will also affect things beyond straight damage. As mentioned above, it'll also affect your mana use. It also equals more critical chance (more attacks = more chances at landing a crit) and a greater chance of triggering certain effects (burn, shock, etc.).