I would judge NPC pay according to PC pay, i.e. let NPCs of roughly equal experience aks for the same amount of gold the PCs would demand when hired. The tables in the DMG for hierling costs only make sense if the hirelings could not do better as adventurers themselves, or in other occupations.
(Mind you, our intellectually-challenged and very naive level 13 Barbarian PC got hired by a PC for 1.5 gold per day as a guard...)
(Mind you, our intellectually-challenged and very naive level 13 Barbarian PC got hired by a PC for 1.5 gold per day as a guard...)