Because of the high cost of getting supplies and equipment delivered, its been costing about $190 per day to maintain a soldier in Afghanistan. For Iraq, its only about $80 a day per soldier. This, despite the fact that Iraq is burning up a lot more ammunition per soldier. The big advantage in Iraq is that the combat zone is close to a source of fuel, which is a major part of the supplies sent in. Equally important, much more material goes to Iraq via ship, being unloaded in Kuwait and driven right into Iraq. There were already some American support facilities in Kuwait, an aftereffect of the 1991 war. These were expanded for the current operations. 

Much more of the material for troops in Afghanistan is flown in. Even the stuff that comes by ship, to Pakistani ports, must then be delivered by truck over atrocious roads. Some loads are lost to accidents, and bandits, which just drives costs up. There many suppliers in Pakistan, and some support facilities (for vehicle and equipment maintenance.) But unlike the Iraq/Kuwait situation, the Pakistani facilities are farther away, and its often cheaper to just fly stuff in and out, or set up a repair shop in Afghanistan.