Is it really possible for someone to find motivation to risk their lives to fight for a paycheck? It would definitely motivate a few but can it motivate a large enough population making private armies feasible?
Rational people, however, may choose to die for a paycheck if said paycheck would help feed their children, parents, friends, and/or significant other. Assuming, of course, that the employer has a reputation for honoring their contract even after you're dead.
Needless to say, people who live in societies with a robust social safety net will be less motivated to fight for a paycheck. In most parts of Africa, on the other hand...
See: pretty much all of history. Generals routinely promised wealth from invasion of neighboring places. In fact, rare is the case where the promise for fighting in a large scale war was not one of wealth, but of some derivative resource ( freedom, religious reasons, pride ). That is a relatively recent construct (maybe last 200 years).
I sometimes feel like I missed my era. Not so good at going in to the office, but might have been a decent warlord. Then I wake up and realize I'd probably just be hung from a tree in any era except this one.