Quite simple, just change this
Code:
soldier          roman_infantry_triarii, 40, 0, 1.2
into this
Code:
soldier          roman_infantry_triarii, 20, 0, 1.2
You could also half the cost and upkeep by changing this
Code:
stat_cost        1, 1524, 381, 30, 40, 1524
into this
Code:
stat_cost        1, 762, 191, 30, 40, 1524
Have fun!