What is the calculation for hiring mercenaries vs volunteers vs drafts? I would guess it depends on whether you have money or not. But it also seems like there

What is the calculation for hiring mercenaries vs volunteers vs drafts? I would guess it depends on whether you have money or not. But it also seems like there are other dangers, like being outbid at the last moment by the enemy and having the mercenaries turn on you. Or maybe desertion, or being too selective about their missions. Maybe mercenaries might even drag out a conflict or misrepresent the conditions in hopes of getting more pay. But mercenaries probably come already trained and with experience, and maybe inside knowledge or even special arrangements to ensure a victory. And what is the calculation for being a mercenary? Obviously you want to avoid becoming cannon fodder, but how do you know?