Yeah, I'm a little worried about Light of the Martyr as well. The ambiguous phrasing is "Sacrifices a moderate amount of your own health to instantly heal an ally for a moderate amount", implying closer to one-to-one. My guess is that the health lost will be based on percent Spell power but will be static, and the healing portion will be based on the same percent Spell power but will be modified by other healing multipliers like mastery, crit, and versatility.
Without any healing multipliers the heal would be zero sum and a net loss of mana, making it almost completely worthless in most situations. Even with multipliers, you would still need roughly 80% combined on crit and mastery (i.e. 40% mastery and 40% crit) to even be worth as much as a single Flash of Light or Holy Light.