About 75 €/month at most, but that would require drinking only specialty coffee. Normally I also have a bag of cheap supermarket coffee, which I use for experiments and training. Really good specially coffee costs about 80…100 €/kg, while good light roasted fresh supermarket coffee costs about 14 €/kg, so that can easily bring that monthly expense down.
Since I drink a little bit of both, I think the overall cost is somewhere around 30…40 €/month.
AP filers are really cheap, so they contribute only cents to the monthly sum. Can you really taste the difference between two filter types? If so, can Chemex really justify the higher cost?