[lch]

With questions like these, it's pretty easy to verify it yourself by starting a testgame with four or more human players, all under your control. Let one hire a merc, then use the others to test any theories.

My guess is the same as voiced before: The one who hired the guys before effectively has his bids doubled in comparison. There is also a nation-specific bonus or malus that might get applied, it's either some percent or half/double the gold again, I don't remember. When there's a tie, then the mercs will either stay with the guy who hired them before, or they'll put themselves up for grabs at the market again, not sure ATM.