Below is a nice little post by forum goer Hugs from Momma that breaks down the cost of Ionic Spark and compares it to the costs of Sword of the Divine and Wit's End, two other attack speed based items with on his procs.
+40% Attack Speed
+30 Magic Resistance
UNIQUE Passive: Your attacks deal 42 bonus magic damage. UNIQUE Passive: Your attacks increase your Magic Resist by 5 for 5 seconds (stacks up to 4 times).
Stat Value: 1513g
Extras cost: 487g
Sword of the Divine
+60% Attack Speed
UNIQUE Passive: Every fourth attack deals 100 extra Magic Damage.
UNIQUE Active: Your attacks cannot be dodged and you gain 30 Armor Penetration for 8 seconds.
Stat value: 1575g
Extras cost: 395g
+45% attack speed
UNIQUE Passive: Every fourth basic attack unleashes a chain lightning, dealing 100 magic damage to up to 4 targets. (400 Bounce Range)
Stat value: 1827g
Extras cost: 473g
Three similar items, all with magic damage procs. Wit's you pay 487 for the 42 magic damage proc and stacking MR. A good deal. SOTD, you pay 395g for 25 magic damage per hit (averaged) and a nice active that has it's place to penetrate armor and counter dodge.
Ionic Spark, you pay 473 for 25 damage per hit that can potentially chain onto 4 enemies. Doing a tiny amount of damage as an AE just isn't a compelling benefit to justify the cost. I rarely see this item built, and it should certainly get another look.
Adding 10% more attack speed, or cutting a few hundred off the cost might make it a useful choice against other, similar options. I want to like this item, but it's outclassed by Lightbringer (for AS and pure utility) and Wit's (defensive stats, magic damage)
Brackhar commented the post with the following feedback.
I think the item bares some re-evaluation, definitely. It's a fun item when at the right power, I just think the numbers are a little off now. Thanks for bringing it up!- Brackhar, Technical Designer, via the official forums.