Of course, in this example, we don't know...
1) What maneuver, if any, the fighter might've been using.
2) If characters get extra damage in place of iterative attacks.
3) What magic weapon the fighter might be using.
4) What level the characters are.
If we're talking about epic characters here (in the 4E sense), a single hit doing hundreds of points isn't out of the question. But that said, we don't even know for a fact that he did 500 in that shot. We just know that's the shot that dropped the dragon below half.