Two fuels can have very different purchase prices and still produce steam at a surprisingly similar cost.
Start with comparable units
A price per tonne of fuel does not describe how much useful heat that fuel produces. Compare fuel prices against energy content and realistic conversion efficiency.
Account for operating assumptions
A useful simplified relationship is: delivered fuel cost per tonne divided by calorific value (GJ per tonne) and by boiler efficiency. The result is an estimated fuel cost per useful GJ.
Use measured data for real decisions
Any reliable industrial assessment must use verified plant conditions, appropriate methods and qualified technical review. Illustrative figures are useful for understanding a concept, not for committing to a purchase or equipment modification.