Are caching proxy servers still be helpful for saving bandwith, now that major services have migrated to https?

I live in Turkey and am part of a group planning to build a WiFi-based ISP to service my home town, as we believe it will be easier to connect all the sub-villages that are not close to our town center wirelessly. During our research about the required equipment for this project, we've read about caching proxy servers that store data from frequently visited websites so that content can be downloaded from the cache proxy server rather than going over our back-haul to the internet and consuming that bandwidth.

We expect that our users will be highly trafficking popular social networking websites like Facebook and Twitter, as well as data and video content farms, online banking websites and many other websites that use https. From all I have read about the subject, we won't be able to cache this content, so I feel like maybe we should give up the search for using a caching proxy server.

In our case, does it make sense to consider using a caching proxy server, considering that we expect most of our traffic to be over https? Will we be able to save significant bandwidth over our back-haul using this technology, or not?

You are right: the caching proxy gives diminishing returns nowadays.

Depending on the browsing patterns it might or might not make sense, but most likely it doesn't. If you look top accessed developed world sites, which generate most traffic, they are definitely behind SSL and caching proxy cannot know (and cache) what goes in the wire.

If you have enough time and resources in hands and what to figure out the exact number, the way to find out is to attach a network monitor and see what part of the traffic is HTTP and what other protocols. The caching proxy comes with some cost (installation, maintenance, resolving proxy issues) and you can (somehow) estimate this and compare this to the price of the traffic.