After killing a monster, Mesos or monster leftovers (ETC. items) may be dropped sometimes. However, the monster may also drop something rare (infrequently of course), such as an equipment, which can be worth a lot. The mesos dropped by a monster is varied, and the range varies with the monster (and is usually, but not always, somewhat proportional to the level of the monster, so you will find that in SOME circumstances monsters of same level drops the same range of mesos), the maximum value usually being 1.5x the minimum value. Monster leftovers have many varying uses.

 

Some are used for quests, exchange quests where large numbers can be turned in to an NPC for experience, or for forging materials, such as Firewood that is used to make Processed Wood. In versions with the Maker skill, 100 monster leftovers can be combined to form a "Monster Crystal", an alchemical reagent used to create items and mesos.