The answer depends on whether you want to work with the whole of a website or just part of it. It also depends on your network setup.

The easiest solution would be to buy more data-transfer from your ISP or choose and ISP that didn't impose such limits.

A fairly easy solution would be to access the Internet through a proxy server which was configured to cache the files that had previously been accessed. This works very well if a number of the employees in your company access the same pages.

You could also mirror the whole or part of the site and maintain a local copy. The initial download will take up a lot of your data transfer limit but (if the software is any good) the update process should only validate the cached files and not download unchanged ones. Check out wget for a start.

Downloading a discrete set of pages should be easy enough with a script. You will have to handle the login and then work through the target pages, following links, downloading target pages and changing the links so that they refer to local files. Look at the section on mirroring in the LWP documentation.