Specific numbers - the cost of carbon

I just read an article http://finance.sympatico.msn.ca/investing/news/businessnews/article.aspx?cp-documentid=4929410 that says Air Canada is offering a "carbon offset", where you can pay for the carbon dioxide your flights generate, to a company called www.zerofootprint.com.

I've read a bit of the zerofootprint.com site, and it seems kind of light on facts (as in "How much of my money actually goes to the carbon offsetting prices?" "Well, we can't say exactly because the price of carbon, as a commodity, is constantly fluctuating. But we have very low overhead, so a lot of your money goes where it should." Still, it's an interesting idea. What I'm most interested in is the fact that they've done estimates of how much carbon dioxide gets emitted per person on a flight. According to ZeroFootprint, an Air Canada flight from St. John's to Vancouver will take about 0.5 tonnes of CO2 emissions, per person. The cost to offset 1 tonne of CO2 emissions appears to be $16.00

By contrast, they say that the average person's share of emissions per year is 20 tonnes.

So... According to this calculation, we ought to be able to offset our total carbon emissions for an annual cost of $320 each. less than $1 per day. This seems a little bit low, considering the hand-wringing over how impossible it's going to be for us to deal with climate change.