This spreadsheet will show you how many BTU's you burn, using individual species as the input. It then shows equivalent amount of oil or propane, and the cost you have saved by burning wood instead. This will rely on input and some basic assumptions from you. The spreadsheet does not compensate for moisture content of your wood, etc. It is a great way to justify expenses.
Uses Excel or equivalent program. Open Office and Libre Office work well and are free alternatives to Microsoft Office.