gives 40 damage, and causes all physical damage to ignore 35% of target's total armor applied BEFORE Armor Penetration. That means that any armor I have, natural or bonus, is reduced by 35% off the bat. That could be a Randuin's Omen, a Thornmail, a Zhonya's Hourglass, and a Sunfire Cape.
Let's make that the example. Say I have built those 4 items. Together they give: 70+100+50+45 (respectively) = 265 armor. That's a lot of armor, but let's pretend they have an AD heavy comp so this is a good idea.
The total cost of that build is: 2850+2100+3300+2600(respectively) = 10850g
I'm level 16, for argument's sake, and I'm Garen. So I have 71 Natural Armor.
Last Whisper automatically ignores 35% of all armor, so 25 of my natural armor, gone. And 93 of my bonus armor, ignored. That's my total armor = 265+71 = 336 - (25+93=118) = 218. sigh sadly, that Armor = more HP comparison for explaining damage reduction /hp/armor thing completely befuddles me.
Anyway, let's say that Of the 10850g for the build, 6000g is just for the armor stat, the other 4850g representing the value of the other stats/passives. now then, 35% of 6000g is 2100g.
Last Whisper costs 2300g, that doesn't look so bad. But if we remove the damage value from LW by comparing it to 4 Longswords (4*360g=1440g) Then you're saying the value of the armor ignore is 900g. So by this highly convoluted comparison, you're removing 2100g value from my build for the cost of 900g in your build.
That was fun, but it doesn't prove much, honestly, due to analoguing stats of items to their gold cost and trying to use that as a comparison when that might not be right. Also, the 6000g value of the armor stat is complete speculation.
Take it as you will, My personal feeling is that the Last Whisper and Void Staff are just a little too efficient against tanks.