Free Exchange looks at the British example:
The latest target is a 34% cut [in emissions], compared to 1990 levels, by 2020.
When the good folk at Cambridge Econometrics fed this requirement into their model, it began spitting out prices of many hundreds of pounds per ton, far beyond anything that could be seen as politically plausible by even the most starry-eyed environmentalist. Indeed, the prices were so high that they were pushing up against the limits of the model, and any numbers generated would have to be taken with a big dollop of salt. The main reason, said the firm's analysts, was the time it takes to do things like building nuclear power stations and converting large chunks of Britain's transport to run on electricity. It just isn't possible to do enough in a decade, no matter how high you crank the carbon price.
The Dish 