I only want half of that so I only want to pay half
I only need half of that so it should be half price
半分 - half
半額 - half price, 50% off
半分のサイズ - half size, half portion
半分でいいから - Half is fine, only need half
〜〜から - therefore, so, because
半額にして欲しい - I want 50% off, I want it to be half price, I only want to pay half
I only want to pay half because I only need half
It should be half price because only half is fine