Well you need the heating value of natural gas and how much or it you will need to generate 1.4 MMBTU/h. Once you have that you need the spot price of natural gas.
A ball park number would be 1.4MMBTU/Hr/ (boiler efficiency 0.8)*$ you pay per million BTU of natural gas.
So if you are paying $2.72/MMBTU then the cost is $4.76/hr or about $3400 a month.