IOS player here so i am a newbie here. But from what i'e read in this thread so far, i can only think of a couple solutions. As mentioned before, Make it so that the blood cost is based upon current Hp, not max. Second, devs might have to tweak the percentage cost. Maybe something like a tiered system. Tier 1 skills cost 2-4-6-8-10% (per level), tier 2 cost 3-6-8-12-15% , tier 3 costs 4-8-12-16-20% and so on.