I thought it was 1d6 per 100 feet, but that may actually come from Holmes or elsewhere. Searching through Vol 3, I only found two references to falling damage.
"Falling into the pit would typically cause damage if a 1 or a 2 were rolled. Otherwise, it would only mean about one turn of time to clamber out, providing the character had spikes or associates to pull him out, and providing the pit wasn't one with a snap-shut door and the victim was alone."and Page 7:
"For example, there is no question that a player's character could easily be killed by falling into a pit thirty feet deep or into a shallow pit filled with poisoned spikes, and this is quite undesirable in most instances."There doesn't appear to be any specification of deeper pits doing more damage, so perhaps 1d6 + saving vs. death or injury, as suggested here, is completely appropriate.
Does any know of a passage I missed?