Are the monster treasure values for a single monster, or for a whole Lair (taking into account all monsters in it)? For example, is the Stirge’s Treasure F for each Stirge, or for their whole Nest of 3d12?
If so, is the treasure value calculated according to the sum XP value of teh average number of monster sin the lair?
As I understand it, the loot is for the whole lair not an individual. Sometimes it mentions that in the Treasure Type stats (such as ‘per warband’). I expect it works out on average, but I haven’t done the maths.
Treasure Type is per lair, except where otherwise noted (e.g. “per warband”).
As for how the values were calculated–some serious math went into these, for which Tavis Allison should be thanked!
Tavis realized that if we simply calculate average treasure based on the xp per monster, it would yield below-the-recommended level because many monsters have no treasure type. He therefore did the math so that the average treasure works out across ALL monsters at different levels.
So, if I want to create a new monster, how do I calculate its treasure?
Use (# in lair) x (XP value) x 4 and then choose a treasure type that approximates that.
Is there a rough % of monsters that are expected to not have treasure? Obviously it’s very vague, since different monsters will be encountered at different rates… did you just go off the total number of monsters, or did you try to account for the likelihood of encountering specific monsters? Are they e.g. balanced by terrain type?
Rhyn, we used the distribution for dungeon levels as our guide, rather than the distribution for wilderness encounters. So the treasure types are balanced on a dungeon level by dungeon level basis.
All right, thanks. That makes sense and makes things easier. I’ve got 40 new or converted monsters for my Dark Sun campaign so far (and I haven’t even started on going through 2E Dark Sun material or borrowing from Caves of Qud), and I’m going to be going through them and fixing their treasure types… for now, I’ll just go with “does it make sense for this creature to have treasure” and worry about balancing them against encounter tables later.
Ryhnn, I hope you share those monsters with the group when you are done. 
Some of them (e.g. war-chimps) are pretty direct knock-offs of other people’s OSR creatures, but I can share a few dozen for sure (husk ghouls, freaks, air sharks, swine-things, vat-men, langford basilisks, and more). I’ll start a thread over at House Rules when I’ve adjusted and edited all of them. My Dark Sun setting is very much inspired by Planet Algol, so they’re a bit “gonzo”…