Out of curiosity I did a minor amount of research to get an idea.

Turns out that you are right, some balloon gas is 80%. Specifically, the "Balloon Time" tanks you can buy at places like Target say "not less than 80%" helium.

On the other hand, I went to AirGas and a few other suppliers and they seemed to have 95%-97.0% helium gas as their definition for balloon grade.