I did some calculations, even if we count just metal output, then it is about 25% chance to get some kind of metal from a scrap box, if using alloy furnace to smelt wagons and helmets.
EU price to make a metal ingot is about 10k with scrap box, and over 40k with mass fab. And as a "bonus" there are leather, diamonds, rubber, redstone, pumpkins, etc,etc,etc.
Of cause, this is a very raw approximate, but even with over-overclocked recyclers scrapboxing seems to be better for, say, mass producing windmills, than mass fab.
Am I missing something?