IAWYC (and upvoted because I don't think it deserves to be that low), but 1) you're being too optimistic about the cheapest way to save lives through charity (according to GiveWell, that's the Anti Malaria Foundation at $1600 per life), 2) the life expectancy at 23 years doesn't equal the life expectancy at birth minus 23 years (life expectancy at birth takes into account that some people die before the age of 23, and you already know she's not one of them), and 3) valuing someone's life according to how much money the average person in their country makes over their lifetime sounds somewhat fucked up to me.