A fall from what height onto a hard surface will result in 50% chance of mortality? 90%?

50% = 48 feet (4 stories)
90% = 84 feet (7 stories)

Knowing this, check this video out: A man survives after bungee cord snaps on 400-foot dive.

While initially sounding quite impressive, the odds of this are well within plausibility. The bungee cord essentially converted a 400 foot fall into one that was only 25 feet.

Source

Marx: Rosen's Emergency Medicine, Concepts and Clinical Practice, 6th ed.