I thought I'd post a few thoughts on this subject. Maybe it will be of use to someone.
I tend to look at how experience works in the game perhaps a bit much, due to trying to analyse how to TR efficiently for more than a year. I have found that there are many misconceptions about how much XP a group lose from a death.
A couple of facts that might surprise people:
The exact figure you lose is the same in whichever run you have a death in the group. Be it the first or the 10th run. The only (slight) difference is when you change difficulty, since the base XP differs.
A death in a first run of a quest actually has much less importance than in subsequent ones if you look at the proportion of the total xp you lose. Not the other way around as one might think. This follows from the above. The main loss is usually time when a death occurs if you're looking at XP/minute.
Some (slightly) simplified formulas:
XP gained from a quest (disregarding optionals and difficulty) = Base XP*(1)*(2)
(1) 1+item (0.05 from Worldshapers Mantle for instance)+Experience Potion+Experience Shrine
(2) 1+All quest bonuses and penalties. This includes death penalty, or rather the lack of the 10% bonus you get for not dying. This also includes, perhaps surprisingly, bravery, tome and world experience bonus.
Run with no death = Base*(1)*(2)
Run with death = Base*(1)*((2)-0.1)
From this and some basic math you can see that the loss from a death in a quest is 0.1*Base XP*(1)
No bonuses (except the ones in (1) above) affects this. Including repetition, firsttime, bravery, tome, world bonus experience and so on.
An example:
Coalescence Chamber first time on elite with full bravery, no deaths, no reentries and 15% other quest bonuses (say onslaught bonus). Gygax trinket, 5% experience shrine and 20% XP potion.
This run will give 43707 XP. One might think that the group lose 4.4k XP from a death there but it's actually only 1305 XP
0.1*10036(base elite XP)*1.30(XP shrine, item and potion)
This can of course be tested by yourself ingame by having all other factors equal between two runs of a quest except for a death.
Caveat: I have not checked things ingame for while, so things might have changed and of course my calculations might be wrong.
Cheers
/G