Looking in the compendium, there do not seem to be rules for this. That said, if you're running Eberron, you might take a peek in the campaign book and see if there is something more specific to the setting. However, there are rules for pocket change and how much a major and minor purchase might cost that are pegged to the level of the character.
In general, I would consider hiring mercenaries (particularly a set who would make a difference fighting alongside adventurers) to be a major purchase, and probably a major purchase per PC. So if your characters are interested in hiring something like Level 1 mercenary thugs, I'd make that a Level 1 major purchase, 144gp.
The major/minor purchase table is in Mordenkainan's Magnificent Emporium, or can be found via DDI.