2 Points for consideration

[Shigawire]

You would hate me ... But still I have to say that there is one problem - in some areas plunder would be very big - in fact many wars were pure plunder which RTW is not depicting.
Making staying on enemy land very expensive won't be historical - in fact (exept some regions - forests, deserts) staying on enemy teritory should in fact give you money, not take it from you. On the other hand, in your land army needs to be fed and paid by your own men ...

also is there any posibility to give money to army winning a battle if anemy is destoyed or routs? It should be scaled to enemy army size, as it is to reflect capture enemy camp, loot the dead, sell captives.

I would hate you?

I don't see why I would hate someone who reminds me of something so important. Loot! We can't have realistic attrition/logistics without local loot! In fact vanilla does simulate this for the faction being looted. The tile the enemy army sits on starts to look "scorched", and you get "pillage" penalties in your provincial finances.

Something you forget though, is that a lot of this money is basically just going into the hands of the soldiers who pillage. Varying on the abilities of the commander, little of this loot actually goes to the state treasury. But that's what traits and ancillaries can help with.

The mercenary army, of which Xenophon was a guest, also went through Cilicia. Due to Cyrus being unable to control them properly, they looted the country and wreaked a lot of havoc. Only thanks to Cyrus' diplomatic skills, and good reputation, did he save this dire situation. You remember the part where both monarchs gave eachother presents to patch up the relationship..

How much of the money do you think went directly into Cyrus' treasury? The soldiers were the looters. Most likely he ordered them to give it all back for diplomacy's sake. And to preserve his good reputation.

Hannibal Barka's forces moving through aquitania and the alps would scarcely get much of a bonus.. considering he lost 80`000 men on the march. But once on fertile land, with good celtic allies, he started getting reinforcements and supplies.. The local loot soldiers take from the areas they march over is small pickings compared to the enormous costs of bringing about sufficient logistical operations. But local loot may help alleviate these costs somewhat for an able commander. This is micro compared to macro though.

Allowing an army to loot the countryside is one way of preserving "morale" (god knows that definition is "flexible")..