I don't know whether you really want to know or not, but carbon credits are not based on the "value" of carbon. Companies emitting CO2 pollution are given a limit on emmissions. They monitor the emissions, and if they are lower than their limit they recieve carbon credits, if they are higher they must purchase them. This allows the company to decide whether it is more beneficial to modify their equipment to less polluting types, or to buy carbon credits. It is a market, so the value fluctuates based upon supply and demand. It is a pretty good system for regulating pollution, and it is far superior to the traditional limit and fine system.