It’s going to cost $6 billion in transportation costs to get American equipment and weapons out of Afghanistan before the end of 2014. After that a smaller (than the current 70,000 troops) force will stay for an indefinite period. Some of the gear to be removed has been there since 2001, but most of it arrived in the last five years. Some $26 billion worth of gear has to be moved, given to the Afghan security forces, sold locally, or destroyed.
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 over 21 months.
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 U.S. built 324,000 aircraft during the war but 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 a lot of rusting reminders of the American campaign lying about Afghanistan for decades to come.