A lair is defined in the monster books. Typically, lairs are found in the wilderness. However, the DM may place lairs in certain special cases, such as dragons. Usually though, it is best to use dungeon treasure in dungeons.

There is a severe contradiction in the rulebooks about amount of treasure in lairs. Monster books such as the MM and MM2 read,¹⁾

Finally, it mustbe stated that treasure types are based upon the occurrence of a mean number of monsters as indicated by the number appearing and adjustments detailed in the explanatory material particular to the monster in question. Adjustment downwards should always be made for instances where a few monsters are encountered. Similarly, a minor adjustment upwards might be called for if the actual number of monsters encountered is greatly in excess of the mean.

In direct contrast, the DMG (published after the MM but before the MM2) reads,²⁾

The TREASURE TYPES given in the MONSTER MANUAL are the optimums and are meant to consider the maximum number of creatures guarding them.

The Demon Idol method splits the difference:

• In cases where the number of monsters actually appearing is less than the average, scale the results of the treasure tables down proportionally from the average (not the max). For example, if the # appearing is 10-100 and 25 monsters appear, since the average is 55, the treasure rolled on the treasure tables will be reduced by 45.45% (25/55).³⁾ Fractions resulting from applying the penalty should be rounded up.
• In cases where the number of monsters actually appearing is greater than the average, do not scale the results of the treasure tables at all.