ruleslawyer said:A-frickin'-men!
Having been DMing Iron Heroes for a while now, I realize that one can't simply slap on a flat level adjustment for differential PC wealth (or CR adjustment for NPC/monster wealth), but at least a detailed explanation would be great!
I find that NPCS with NPC wealth are consistently about 3/4 the CR (rounded up) of PCs with PC wealth. NPC spellcasters' effective CR of course varies a lot depending on whether they've buffed up, but this 3/4 rule is overall extremely consistent. An NPC Ftr-3 is as good as a PC Ftr-3, but by 20th level the NPC Ftr-20 is far weaker than the PC Ftr-20.
If you half XP, I recommend don't generally half NPC wealth, the poor guys are already hard done by. If you did do so, though, I think NPC CR would fall to about 2/3 of their level.