Between $75-$120 a month, for two of us.
We pay an average of $22 for a 12 oz bag of the good stuff from local roasters (Temple or Chocolate Fish) and go through just over a bag a week. Work days we have 2 cups each, weekends we go all out and have 3. We buy whole beans, grind them at home, and exclusively use the Chemex. Sometimes when we pick up a bag at the shop we treat ourselves to a latte (stupidly expensive) and that bumps up the average.
I love this! For anyone wondering, the author is Ian Hamilton Finley from his book Glasgow Beasts, an a Burd, Haw, an Inseks, an, Aw a Fush.