150% bonus damage with 20% chance balances out to 30% bonus every auto, compared to Deathcap's +35% AP every spell. However very few spells have 100% AP Ratios to take full advantage of the +35% AP, usually having around 70% ratios, resulting in 25% bonus damage for that ability, making IE's bonus larger on average than Deathcap's.
The advantage Deathcap has over IE is that it's consistent, which is what pushes IE ahead late game as the player gets more and more Crit Chance items. Instead of 1/5 chance it gets bumped up to a 1/2 chance with as little as one other item, making it an average 75% damage boost per auto.
Just when I thought I've seen it all people actually ate attempting to say IE is the same as deathcap holy shit.
Same, no, but it is the equivalent in the same way that Last Whisper's upgrades are the equivalent of Void Staff or that Mercurial Semiter is equivalent of Zhonya's Hourglass.
+80 AD
+35% Total AD
3800g
If you want a real comparison, all other AD items would drop down to 55 AD, it would only work with Bonus AD (no free AP per level, no free AD per level), it would cost more than a second BF Sword to combine, and it wouldn't help your Auto attacks without purchasing another item.