You guys are forgetting one feature here, which is also described in the faqs, the growth balance script is taking away money from the player for each settlement based on it's development level by default.
For example: (i have made some code bold for easier viewing)
Code:
monitor_event SettlementTurnEnd FactionIsLocal
and SettlementType city
and SettlementBuildingExists = wooden_pallisade
console_command add_money -100
end_monitor
monitor_event SettlementTurnEnd FactionIsLocal
and SettlementType city
and SettlementBuildingExists = wooden_wall
console_command add_money -200
end_monitor
monitor_event SettlementTurnEnd FactionIsLocal
and SettlementType city
and SettlementBuildingExists = stone_wall
console_command add_money -400
end_monitor
So if you want to change that you can adjust it in your campaign script file, located in the consecutive campaign folder (imperial, no garrison , vanilla)
You should take into consideration though that every change you make does take away from the basic concept of DLV though, being harder than the vanilla game and more challenging.
Edit: One way to see what costs you have right now, is saving your game, then gifting all your money to a faction, then hit end turn and you will see exactly how much you go into negative. then you can reload and act accordingly. Granted that isn't a very handy method but it works...