If you can collect it, you can track it. Collection is your challenge.
But IIS log files *will* record kbytes sent and received automatically. Then any analytics program, including WebTrends or even Excel, can summarize that log file field for you.
Apache logs will collect the KB number also, but WebTrends will not recognize the kbyte field in Apache logs (mostly because Apache logs don't label their fields the way IIS does.
So, bottom line- get your log files. Have WebTrends do the analysis if you want, and if you have the WebTrends software of course. (If you use the ASP model, it won't work). And it wouldn't be very hard to write a simple script that will process the logs, adding up all the values in the kbyte field. You won't need to try to clump the hits into visits to get bandwidth usage, so that makes the processing super-simple.