I honestly think that talking about both numbers should be doable even for a brief conversation. Many things are very cheap to provide to people (e.g. a hot meal), so it seems like an intervention being low-cost wouldn't be too compelling on its own. You could try working the figures in together, like so:

"It costs about $5 to provide medication to one child. Normally, that just (describe the normal effect of the medicine; reducing symptoms?). But sometimes, it actually saves the child's life. Research shows that it only costs about $2300 to save one child from dying." (This may sound too "certain" and not hedged enough, but I think you sometimes have to make allowances for that if the conversation is very short.)

If I'd done more research on Malaria Consortium, I'd also try to figure out what "avert a death" or "save a life" means in this case; I'd like to be able to say that every $2300, on average, probably leads to one child living a long and healthy life who would otherwise die, but I don't know whether that's what the number represents here.