I've seen this item go for cheaper that it should and I've also seen spell penetration items go cheap too - and it is because (imho) of a general gap in people's knowledge of spell pen.

I'll kick off with a question. "How much does spell penetation reduce damage".

The answer is - with maximum resistance (300) you will get an average of 75% damage resisted - and it's a sliding scale.

So 150 resistance = half of 75% = 37.5%

Average Resistance(%) = (Actual Resistance / 300) X (3/4)

So 10 points of magical resistance will give you "on average" 2.5% more resistance. And it will give a mob 2.5% more resistance too.

Given that CoS/CoE reduce mobs resistance by 75 - you really need to encounter a mob whose resistance > 85 in order to get the full benefit from this item. Fortunately much trash and most bosses do have resistance over 85.

Now - let's think about this. If you are hitting a average shadowbolt for 1000 (that's quite low) then you will get 25 extra damage.

Now don't forget +dmg gear is affected by the "casting time ratio rule" - in the case of shadowbolt : (6/7). How much extra "+dmg" would you have to have to get +25dmg from a shadowbolt ? Well it would be 25*7/6 = 29.2


Conclusion.

If the mob has over 85 resist then "10 spell pen" is equivalent to "+29.2 dmg" for every 1000 your average shadowbolt hits for.

I don't have to tell you that if your average shadowbolt is 1500 - then you're gonna get alot more benefit.