Note: This procedure requires editing a game file; creating a backup copy of the file before proceeding is recommended. With a text editor, edit the "descr_mercenaries" file in the "\activision\rome - total war\data\world\maps\campaign\imperial_campaign" folder. Find the line that appears as:
unit merc barbarian infantry, exp 0 cost 750 replenish 0.06 - 0.19 max 2 initial 1
Change only the cost by setting it to "1", so the line now appears as:
unit merc barbarian infantry, exp 0 cost 1 replenish 0.06 - 0.19 max 2 initial 1
Do the same to all mercenary units. Note: Do not add mercenaries in new places as it will prevent you from ever playing the campaign.