It's not entirely clear what you're asking. Two possibilities, corresponding to my above distinction, are:

(1) What (perhaps more general) normatively significant feature is possessed by [saving lives for $500 each] that isn't possessed by [saving mosquitoes for $2000 each]? This would just be to ask for one's fully general normative theory: a utilitarian might point to the greater happiness that would result from the former option. Eventually we'll reach bedrock ("It's just a brute fact that happiness is good!"), at which point the only remaining question is....

(2) In what does the normative signifiance of [happiness] consist? That is, what is the nature of this justificatory status? What are we attributing to happiness when we claim that it is normatively justifying? This is where the non-naturalist insists that attributing normativity to a feature is not merely to attribute some natural quality to it (e.g. of "being the salient goal under discussion" -- that's not such a philosophically interesting property for something to have. E.g., I could know that a feature has this property without this having any rational significance to me at all).

(Note that it's a yet further question whether our attributions of normativity are actually correct, i.e. whether worldly things have the normative properties that we attribute to them.)

I gather it's this second question you had in mind, but again it's crucial to carefully distinguish them since non-naturalist answers to the first question are obviously crazy.