By what I know of it (which, granted, isn't all that much) the Greek city-states generally had very little in the form of taxation and hence a bit of a lack of liquid assets they could invest on something or other - like paying full-time professional troops. Wasn't the hoplite system a specific answer to that - each man obliged to equip himself at his own expense and fight for the community as needed, the costs to the community proper hence about a flat zero ?

'Course mercenaries don't nearly always actually need to be paid as such; they may well be perfectly happy at settling the matter themselves by straight looting of enemy territory, but that's not exactly a workable way to go about maintaining any kind of standing force...