Sounds like you want to parse the squid log, keep track of which IPs are hitting it where, and how much data they're getting. Of course, you'll have to keep track of how big the items in the cache are, since squid doesn't report file size for cache hits. But it does when it loads a file into the cache, so you should be able to store it in a hash of some sort.

Hope this sends you in the right direction!