I agree about reducing the %penetration idea since looking at some champions stats while spectating:
Average Tank Health: 3000-4000 (since they also building Armor and MR)
Average Tank Armor: 200-300
Average Tank MR: 150-200
LW with 35%Armor penetration usually almost worth as 70-105 Armor penetration (60-90 for 30% and 50-75 for 25%)
and Maximum amount of flat Armor penetration stack in current state is 50 for Jungler Warrior (10), Brutalizer (10), Youmuu (20), and Black Cleaver (10)
Void Staff with 35%Magic penetration usually almost worth as 52.5-70 Magic Penetration (45-60 for 30% and 37.5-50 for 25%)
and Maximum amount of flat Magic penetration stack in current state is 30 for Sorcerer Shoes (15) and either Haunting Guise (15) or Liandry (15) since those two can't stack
but that was against tank...
those who doesn't build tank usually only have 50-100 MR (including runes and masteries) because they don't gain MR per level so 17.5-35 Magic penetration (15-30 for 30% and 12.5-25 for 25%)
and their Armor usually between 100-150 so 35-52.5 Armor Penetration (30-45 for 30% and 25-37.5 for 25%)
maybe for LW just reduce the Armor penetration to 30% and raise the price to 2550 (or more price with less %Armor penetration)
and for Void Staff reduce the Magic penetration to 30% and raise the price to 2395 (or more price with less %Magic penetration)
if those stayed at 35% LW should cost minimum 2750 and Void Staff should cost around 2440-2560 but that will result to high combine cost... (minimum 1515 for LW and between 1145-1265 for Void Staff)