(Continuing my example from above)
Let us say that of the 70 damage you got, 50 of it was from attacks targeting AC. Let us say you took a feat that increased this AC by 1 and that enemies hit you 50% of the time before you took the feat.
That means that the base damage they dealt was: 100 * 50% = 50 damage
With one higher ac the damage dealt would be: 100 *45% = 45 damage
Toughness would in other words be about as much worth as one point of AC because of the healing surge extra, and would give you 5hp extra before you went down.
Oh, and remember you die at -50% hp. In the above example, the character with toughness would die at -16, while the other would die at -13.