I would suggest that you've disproved your own argument that knowing how much bandwidth is required doesn't tell you how much data will be used. You simply multiple the bandwidth by the amount of time you want to reference and come up with data use for that time period.
I agree with your math but I think it worth mentioned that the 2.16 Gb you calculated is gigabits. Data plans are typical in bytes, not bits, and the 2.16 Gb is .27 GB, or gigaytes. It may be more meaniful yet to state that as 276 megabytes per hour.
Now that we've crunched the math, let me speak of real world experience. The fact is that video call data use will vary tremendously depending on a numer of factors, those including actual bandwidth available and both ends of the call, the quality of the connection between the contacts on the call, the specific webcams in use and the amount of processing power that's available.
When I'm on a 1920x1080p HD call the typical bandwidth used is ~650 kB/s. A 720p call will run around 350 kB/s, and a good 640x480 SD call ~150 kB/s. I've also had calls where conditions only allow 320x240 and data used can be as litle as 15-20 kB/s. You can do the math if you want data use per minute or hour.
Here's the bottom line....the bandwidth requirements outlined by Skype will tell someone what kind of results they can expect with bandwidth they have available. You can't expect HD video if you only have a .2 mb/s upload connection to the Internet. But to know how much you actually use, I suggest a utility like DU Meter. It will let you monitor your data throughput real time, and also keep track of how much you've used over a given time period.
Let's see....thus far this month I've used 35.64 GB upload @ download; 2.11 GB in the past 24 hours; and 12.8 mB so far today. It's clear as a bell. 
