AI's love of Mercenaries

While working on my mod I have decided to institute AOR (area of recruitment) to ofset this I made mercenaries available for hire in setlements. However, for some reason the AI always hires mercenaries first, regardless of cost-effectiveness. I have mercenaries cost half-again that of the factions own unit with the same stats, armoured sergents and mercenary spearmen for example, but still the AI only uses mercenaries. Dose anyone know a way to make the AI hire thier own units first and mercenaries as a last resort?

Also, dose anyone know the actual formula the AI uses to determine cost-effect of units?