Hiya.
This is going to be all over the map, posting wise. So many variables based on player/DM preference...but anyway...
For me, I see PC death as a guaranteed outcome if they go into "a dungeon" (adventure; whatever...) and just hope for the best. Dungeons (and I'll use that term for any "opposition force" against the PC's, be it actual dungeon, wilderness travel, mad arch-mage villain, neferious slavers, killer thieves guild, etc...they are all doing their own thing if left alone, but once the PC's start to go against/stop them, they *should* be trying to KILL them or otherwise remove them from the playing board called Life). Ahem...where was I?
Oh yeah...for percentages...
Unprepared to go into/against a Dungeon/Adventure... 99%
Semi-prepared...50%
Prepared...25%
Over prepared...10%
And using your example...
...during an encounter (of "equal toughness") 20% average I guess
...during an adventure (of "about 20 hours length") 40% average
...during a campaign (of "about 2 years play") 80% average
In my games, the players know I play the situation as warrented. If the players make an assumption that the Temple of the Howling God is going to be infested with were-wolves, gnolls and all manner of wolf-like creatures...so be it. When they get there and find out it's actully infested with a savage and semi-intelligent race of bipedal howler monkeys who worship a twisted version of the Aztec fire god Huehueteotl...tough noogies. They aren't prepared for it, go in half-blind, and half of them die...*shrug*.
I am a rather harsh DM. I like to think I am fair, and neutral, and I'm pretty sure I am (been at this DM'ing thing for over three decades and my players tend to stay in my games). I give the players an even break, but I don't coddle them. They are playing adventurers by default, *not* "heroes". The title Hero isn't something a person is born with...not in my games. If they want to be heroic, they have to actually *do* heroic things and *be heroic*.
Bottom line...character death should be 80% likely unless the *player* is smart and a bit lucky.
^_^
Paul L. Ming