Unlike Iraq, where heavy stuff, like armored vehicles and trucks, could simply drive to a nearby port and be put on a ship, Afghanistan has no ports. The nearest ones are in Pakistan and the road trip is expensive and dangerous because of the theft and the threat of attacks (by terrorists or gangsters seeking “protection” fees). So a lot more gear will be flown out of Afghanistan, which is quite expensive. The current plan calls for 28,000 vehicles and 20,000 shipping containers of gear are to be moved by the end of 2014.
The U.S. and NATO supplies coming in (or going out) via railroad from Western Europe go through Ukraine, Belarus, Russia, Kazakhstan, and Uzbekistan, to Afghanistan This approach costs $400 a ton to move material to or from Afghanistan, versus three times that to truck it in from Pakistani ports, or $14,000 a ton to fly stuff in, or $10,000 a ton if you just fly material in from a friendly (Persian Gulf) port. For example, $600,000 MRAPs (Mine Resistant Ambush Protected) cost $140,000 to fly in from the Gulf. Some 2,000 of these MRAPs in Afghanistan are no longer needed by the United States or the Afghan forces, so they are being cut up for scrap in Afghanistan.
It's common for troops to destroy, or simply abandon, unneeded military equipment when they return home from a distant place. This occurred on a spectacular scale at the end of World War II, as U.S. forces returned home from just about every part of the planet. Thousands of trucks, combat vehicles, and aircraft were just left behind, some in remote areas where there was no one to use them. The U.S. Navy dumped perfectly good aircraft off the decks of aircraft carriers.
The scale of aircraft abandoned after World War II was staggering. The U.S. built 324,000 aircraft during the war and needed only about twenty percent of those for post-war uses. Most of the 24,000 transport aircraft built were in demand after the war, either for civil or military use. Some of the DC-3 transports are still flying in out-of-the-way places. Some American equipment in Afghanistan will find non-military work. Many of the trucks, for example, are just the sort of rugged transport needed in Afghanistan.
After World War II the 2.8 million trucks could be used in the civilian economy, but the 88,000 tanks and 257,000 artillery weapons could not. Many were disabled and left behind. Much of the stuff was transferred to local allies, but there was sometimes so much surplus that a lot of it can still be found sitting in a jungle, remote forest, or on the bottom of an ocean (or even lake or river). Occasionally an old tank will be discovered underwater in a lake or swamp and dragged out, well preserved and appearing ready for action. The U.S. hopes to avoid this in Afghanistan, but there will no doubt be lots of rusting reminders of the American campaign lying about Afghanistan for decades to come.