I started a couple games (poor me, too busy to play!  

 ) and always thought the mercs were way too expensive, so I never hired them.  But when I checked the next turn to see if the price had gone down, the AI had snapped them up.
Are the merc prices always for a 3-month contract?  At any rate, from what I see, the AI is grabbing mercs ASAP... but these games both had many AIs, at every difficulty level from easy to impossible, so for all I know, maybe some of the levels don't grab mercs correctly.
-Cherry