It would be a tiny, tiny amount (and helium is pretty expensive).
IIRC, Porsche filled the spaceframes of some of their race cars with helium. Not to reduce weight (although you'd wonder why they didn't use something else, Argon, maybe, or even Nitrogen) but so they could, by measuring the pressure, tell if the welds had cracked. There were (maybe still are) some pretty crazy people working for Porsche.