Buy-back centers differ in that the cleaned recyclates are purchased, thus providing a clear incentive for use and creating a stable supply. The post-processed material can then be sold. If this is profitable, this conserves the emission of greenhouse gases; if unprofitable, it increases the emission of greenhouse gases. Government subsidies are necessary to make buy-back centres a viable enterprise. In 1993, according to the U.S. National Waste & Recycling Association, it costs on average $50 to process a ton of material, which can be resold for $30.