Need to get to the desired level
It is the best way to save money while going for 99 fires making, and also the cheapest way to master fire making: 1-15, burn normal logs. 15-30, burn oak logs. 30-99, burn willow logs. Of course, it would take some time, but it is the cheapest way. Estimated amount for using this method is about 1.7m-2m. It takes 144,828 willows for 30-99.
You should fire make at Tabula Rasa Gold if you want to save money. You can buy willows from the general store there for a cheap price. Now we are going to give you the amount of logs needed to get to the desired level.