Gold. Lots of it. You should be able to set your price as a mercenary, because that's what a merc is - a soldier for hire.
To make this work with random BG Qs, every player in the Q should be asked how much gold they are willing to pay to hire a merc and lower their Q times. If it's 10 people willing to pay 100G then you'd be able to hire a merc for up to 1000g. If you needed 3 to fill the team, then you'd have budget of 333g per merc. Minimum merc bids should be imposed, and the option to join a non-merc Q that costs no gold (but may have longer wait times) should exist.
In addition to this, there should be an exchange where group leaders can individually choose and hire merc players for gold. Sort of like an auction house for mercs. I say all this knowing that pvp in 7.0 is looking pretty bad with the changes they're making, but in the off chance that they come to their senses and don't find a way to make pvp even worse than it is now, I would very much enjoy being a merc.