Talk:Kassadin/@comment-25247575-20150426222537/@comment-4636093-20150426223545

Damage reduction's value increases exponentially as you gain more magic resist.

By some coincidence I already did a comparison of 15% damage reduction (actually for Katarina's Shunpo which is thankfully the same).

TL;DR: At 30 magic resistance you can buy about 35 magic resistance to equal 15% damage reduction. At 130 magic resistance you'll need to buy 120 magic resistance to equal 15% damage reduction.