Like Torben said, you realllllllllllllllllllllllllllllllllllllllllllllly need to be constantly a demi-god in battles to even EVER TALK about mercenary paiement.
Its not only because everyone is a veteran in strat battles (mostly) and nicely skilled, but also because it is an additional strain to the management to have to individually monitor and pay guys that does good.
Basically if you constantly have a OP K/D (or are incredibly useful, like skilled catapultist or engineers) and that you manage to make everyone love you as a neutral mercenary (quite hard with everyone shitting on you as soon as you apply for someone's enemies) you should be able to ask for paiement if you know whom to speak to.
The fact that the mercenary paiment is poorly implemented into the game not only created that situation where it is almost non existent to pay mercs, but also that almost everyone stopped caring a long time ago and doesnt want to talk about it since it seems completely exhuberant to do that now.
But I'm sure a gifted individual would do good with the proper contacts if he really wanted to become a pure mercenary.
I would love to see guys succeeding in this role, or even groups of people (or even myself, who doesnt want to be famous & pro?
)