I know it seems low, but I did a quick campaign with Pontos which is one of the smallest factions (has only one region), and I saw that I could survive with this field costs. Of course that bigger factions won't feel that much impact, however, if you have two armies besieging two different settlements for five turns and also another army in enemy lands for the same amount of turns, in five turns you will have spend 5000 mnai, just on field costs. I'm also thinking about expanding the script for the later game with inflation costs in which after the player gets 30 settlements, the costs double.
To modify the file, you need to open it with notepad++ and search for field costs, you will find this:
To increase the field costs just change the numbers to something higher and to replace which factions don't have field costs just replace the name f_faction with another.
Other Faction names:
f_gandhara
f_sweboz
f_pergamon
f_seleukid
f_boii
f_numidia
f_saba
f_arverni
f_aedui
f_carthage
f_parthia (Phalava)
f_hayasdan
f_pontos
f_ptolemaioi
f_baktria
f_arevaci
f_lusotannan
f_rome
f_makedonia
f_bosporan
f_getai
f_casse
f_kh
f_epeiros