Unit costs are all in the export_descr_unit file in the data folder (usually in C:\Program Files\steamapps\common\Rome Total War Gold\data if you have the Steam version). For each unit, the second to last line is stat_cost, which has:

-Number of turns to build
-Cost of unit to construct
-Cost of unit to upkeep
-Cost of upgrading weapons
-Cost of upgrading armour
-Cost for custom battles

All separated by commas. All you have to do is change those values to change unit costs. As with any modding, always make sure to make backups.