The biggest problem is that Skype tunnels a proprietary protocol over port 80. You would have to filter out the Skype traffic from the HTTP traffic going over that port (assuming you are trying to measure Skype bandwidth usage independently from all of your other network usage).

I should have written the number in bits to be clearer, but the value is the same. I wrote it in bytes instead of bits only because the Skype documentation also listed the bandwidth in bytes.

While VoIP calls with a G.729 codec are about 8 kbps, calls with a G.711 codec are 64 kbps. It turns out that when you are doing speech recognition, you can get much better results with a G.711 codec, especially for short utterances. But, for ordinary VoIP calls between two people, G.729 works quite well.

I’m still puzzled by why Skype eats up so much bandwidth, unless less it is true that they are using the extra bandwidth for their own purposes. If you don’t think that is possible, I suggest you carefully read their Terms of Service.