This is % Tactics -

(Images from German version)

Let say if you use Ghost kill as your tactics then % tactics is at lv1 then you will get 5% increase in ninjas on a particular stats and at lv2 10% and it increase accordingly from next level.

and here is the cost -

as youcan see from above image lv1 to lv2 it takes 70 attempts, and each attempt takes 5 particular materials for that tactics. so it will take 70 x 5 = 350 materials and each material's cost is 31 cp.