Ubuntu Installation :: Lucid - /var/cache/apt/archives ?

A few releases ago things used to work this way, but seemingly since Jaunty it no longer does seem to work:

This used to work: I have several Ubuntu machines to upgrade and limited (and highly expensive) bandwidth available. In the past I would upgrade/update one machine and copy the contents of /var/cache/apt/archives to the other machines to be upgraded. When updating/upgrading the distro the packages that already exist in /var/cache/apt/archives would not be downloaded from the Internet while utilising the locally cached .deb files, saving time and bandwidth.

The Problem: I have noticed that since 9.10 (and with 10.04) this no longer appears to work. While the .deb packages may exist in /var/cache/apt/archives, the same packages would be downloaded from the Internet regardless of the same .deb file existing in /var/cache/apt/archives/.

Question: What is wrong and how can I restore the functionality present in copying cached packages from machine to machine in order to save bandwidth?

Why I need this to work: At work (and I have no control over this...) we are limited to a total bandwidth of 3Gb per month for multiple users. Needless to say a single upgrade of a recent Ubuntu distro can decimate a large proportion of our monthly available bandwidth. Upgrading multiple machines is absolutely out of the question if the packages have to be downloaded every time.

An apt-cache server is not an option: The option of a local apt-cache server is not feasible due to the same 3Gb bandwidth constraint. An apt-cache server requires 15Gb storage per version of Ubuntu and the downloading/upgrading of 15Gb worth of packages for that storage is not an option due to the 3Gb limitation.