I was looking in coreunitmonstertypes.xml. I noticed that trained monster units like ogres, trolls, and storm dragons have the line:
<HasWages>0</HasWages>
Whereas Knights of Asok and Hunters (the units that come merc camps) have this line instead:
<WageOverride>0</WageOverride>
Is this why Hunters and Knights of Asok still cost wages?
Maybe there are deeper hard code reasons behind this, maybe not. Hope this observation is helpful.
