title_body: What do producers usually mix ground coffee with to make it cheaper?
upvoted_answer: I will not go into any political discussion, theory or similar, but I can give you a few historical facts.
  
  There have always been times in the last few centuries when coffee was a rare commodity. And it has always been expensive to some degree, making it "something for special occasions" in the poorer parts of European societies.
  
  People used and still use various "substitutes" from grains (barley, spelt, rice), roots (chiccory, dandelion), fruit (acorns, dates), typically roasted, ground and prepared like coffee beans, resulting in a somewhat bitter, dark drink more or less resembling genuine coffee. (For a more extensive list, see the link above.) 
  
  Instead of replacing coffee, they can also be mixed with coffee, "stretching" the yield of the coffee beans.
  
  If you want to check your coffee for "fillers", buy whole beans - none of the coffee substitutes look even remotely like coffee beans.