Is there another name for marginal luxury?
The closest I can think of is Stone-Geary utility/preferences, where consumption is assumed to provide utility only above a specific level (the $\gamma_i$ parameter in the link), which is usually designated as the "subsistence level".

As for how much an economy is above this subsistence threshold, a way to roughly assess this is to calculate what would Total Income be if all people were at the poverty-line-income (each country determines its own poverty line). Then subtract this from the actual Total Income. What is left can be thought of as what the OP is looking for.

But calling it "luxury", implies that any consumption above the level guaranteeing mere survival is a luxury. Is it?