Depending on how your house is set up and how much you use the stove I suspect you will save at least 30%. Most likely MUCH MUCH more. I have heated this house for 11 years with 100% wood and last year put in a coal stove, a little Chubby Jr. It was used 90% of the time and the Soapstone Woodstove 10%. Even though I am for the first time in years paying for a fuel source (coal) I am elated that I am.
Of course there many variables are in the mix when it comes to actual savings. There is a little learning curve when it comes to burning coal but if I can figure it out anyone can.
Don't be surprised if your payback on the stove and the setup happens much faster than you think it will. You have made a great investment and that investment will put money back in your pocket. And that is always a nice thing to have happen!
Keep us posted on your progress and EVERYONE around here LOVES pictures... (hint hint)