You can find prices for carbon credits (US) on CCX here http://www.chicagoclimatex.com/market/data/summary.jsf (currently trades around $2 per metric ton). CO2 stats is buying wind energy credits which are different than CFI traded on CCX so they could be paying a different price, but it's unlikely to be materially different (because CFI is basically a listed version of wind credits, bio fuels, land fill gas to energy, etc.). So if they have enough volume, this is the price CO2stats should be able to buy offsets.
In terms of the price at which they sell, it varies on a per ton basis because of their fixed monthly fee structure. http://www.co2stats.com/signup.php $10 per month gets you 100K page views. Based on the stats from my blog, CO2stats equates 100K page views to approximately 500 kilos of CO2. So the cheapest they sell at is $20 per ton but the reality is that it is much higher because most clients wouldn't have 100K page views. For example, if your site gets 10K page views, then you'd be looking at $200 per ton.
In terms of the price at which they sell, it varies on a per ton basis because of their fixed monthly fee structure. http://www.co2stats.com/signup.php $10 per month gets you 100K page views. Based on the stats from my blog, CO2stats equates 100K page views to approximately 500 kilos of CO2. So the cheapest they sell at is $20 per ton but the reality is that it is much higher because most clients wouldn't have 100K page views. For example, if your site gets 10K page views, then you'd be looking at $200 per ton.