There’s no final decision on this yet, but the upkeep will probably work twofold. One is a normal gold upkeep for each soldier (according to his level and background) the other is the food he consumes. Food can only be bought in villages. So before venturing out on a long exploration you have to stock up on food. On top of that you have to protect the villages, because when they get pillaged and destroyed you won’t be able to buy your food there anymore.
