From what I've read it seems most soldiers did not get payed until the end of the campaign/term of service. I guess it was a devious way to make money, as the soldier had to survive the term/campaign to get his paycheck. Also it was a bit of an incentive for routing soldiers to rally back at a city or fort.

Very few soldiers were professionals recieving a monthly/quarterly or annual pay

Could make quite an interesting campaign if at the end of your conquering you were short on the loot to pay your soldiers.

And the mercanary siege of Carthage was because they only got half pay, not because they did'nt get payed wich seems like what most people refer to.