When in fight, I got millions of Toughness.
When testing it with dummies, I got about 22 Toughness per point in max life (for armor, resis, DR), but I got 625 Toughness per point in Barrier (linear from 0 to 100% barrier).
I think this numbers should be the same.
So if I get to 250k Toughness without Barrier, I should get to 500k Toughness with 100% Barrier.