Okay, I'll try. Regardless of level, if you take damage down to -10, you die. But if you have one-hit-point toughness for an extra hit point, you would be at -9 and you live. If you don't die, you don't lose a level upon being raised. So one-hit-point toughness is like True Resurrection without the GP cost. But it is better than True Resurrection because it is basically an immediate action because you don't wait to come back from the dead because you never died.
But wait, there is more, if you are hit to exactly 0hp, you can only take partial actions and if you act, you take damage. On the other hand, if you have one-hit-point toughness, you would have 1hp and could take a full round action. Turning a partial action into a full-round action is something only 3.0's broken haste spell could do. But wait, there's more: You can act all day long and never take damage. A spell that prevents damage every round all day long, I shudder to think what level that spell would be.
So toughness even at one hit point is equvialent to True Resurrection, 25,000 gp, an all-day version of 3.0 haste, and some kind of all-day damage prevention spell. All quickened and uncounterable. With that kind of brokenness, who would possibly think toughness should be worth any more?