Wouldn't it be better if you had?:
5 Healing Spray S = 1 Healing Spray M
5 Healing Spray M = 1 Healing spray L
That way, you'd rely on healing spray S throughout the beginning of the game as you don't require that many. Later in game you will find it a little more often, but if you just require 2 to upgrade it, then it would be too good.