What does REDD+ really cost?

What does REDD+ cost? At least since the influential Stern Review was published in 2006, many have argued that REDD+ (Reducing Emissions from Deforestation and forest Degradation) is one of the cheapest options to mitigate climate change. Others see the REDD+ mechanism as a costly effort with unpredictable results for the climate and forest peoples of the world. Who is right?