A lot of factors can change the #/day.
You can throttle it down to probably 1#/hr (maybe less) or up to 3-5#'s/hour depending on how much heat is needed.
Say 1#/hr or 24#/day min setting = $2.50/day x 30 days = $75/month (Assuming $5 = 50#bag)
Say your normal Heating bill is $250/mon and now you only run it 1/2 the time with the Coal, that's Saving $150 - $75 for coal = $75 saving for one month.
Every heating situation is different. Also depends on how much heat you get upstairs to satisfy your other heating sytem, if you can keep it from coming on, you are already saving money.