<p>Can I determine from an ASP.NET application the transfer rate, i.e. how many KB per second are transferd?</p>

There are a number of debugging tools you can use to check this at the browser.  It will of course vary by page, cache settings, server load, network connection speed, etc.
Check out
http://www.fiddlertool.com/fiddler/
Or if you are using Firefox, the FireBug add-in
http://addons.mozilla.org/en-US/firefox/addon/1843