Provide a cacheing web proxy, with lower charges for accessing cached objects

You supply a caching web proxy, which customers can opt-in to use. This proxy could be used without authentication by any device connecting to the internet via AAISP.

Data usage for objects served directly from cache are charged at a lower rate than normal data usage charges, to reflect the reduced use of upstream bandwidth.

Could potentially be extended to proxy e.g. bittorrent.

(I don't know that this would actually reduce costs, because e.g. I don't know how expensive your upstream bandwidth is compared to your other expenses and I don't know your network topology. But it's an idea.)