I have read that collectively, NATO will be abandoning equipment with a value of up $2 Billion.
Not exactly the best of news especially for nations caught in the grip of austerity!
The reason given is that it costs too much to bring it home, which is rather a surprising claim given the cost of most military equipment these days. There is very little doubt that the cost is either being raised by punitive tariffs on the nations through which they would need to transit, if not actually banned.
It does create a PR disaster as well, as the overall image: Enemy still in the field, troops leaving by the shortest route and weapons left abandoned, is one as old as warfare itself.
NATO has been trying to mitigate that image by present the equipment as military aid to some Central Asian countries, but Russia has objected very strongly to this, which makes pretty clear that it is determined not only to control what equipment leaves Afghanistan but also the way it is disposed of.
I also strongly suspect that the awarding of Afghanistan by the US as "Key none NATO ally (or somesuch)" the other week along with the billions worth of aid, is a package to help the disposal of the equipment in a dignified manner.
Much of above was being discussed while only the Central Asia, Russia exit route was the only one available. Since then Pakistan has reopened its routes, which reintroduces other factors, but of course it is politically precarious and only ever one drone strike away from being closed again.