Random generator gave me Elixir of Vitality and Wind Drake. I'm not sure if this card works here. I also thought of making a griffin that gained 4 life when it entered the battlefield and gained 8 life when it left the battlefield. How expensive would that 2/2 flyer need to be to be fair? ? Would it have been a better card? I don't know..

Naah. That flyer you describe could easily be 6 mana; perhaps even 5. Contrast Aven Riftwatcher, Angel of Mercy, and Shu Soldier-Farmers. The total amount gained is higher, sure, but I think it'd be okay at 6 mana.

That said, this is a fun design too.

It's funny, because I think you're closer to right than I am. Despite the fact that a sorcery that said "Gain 12 life" would probably cost (assuming uncommon or common. Rare might cost , the way Wizards has been differentiating lately.). I'm not sure, though. They haven't printed anything like that in a little while. Life Gain has been all over the map. Angel's Mercy, Heroes' Reunion and Primal Command, Rest for the Weary and Natural Spring all seem to have different takes on how much 7-8 life should cost.