Fungus in furnace - Air Duct Cleanning

We scheduled air duct cleanning and upon furnace inspection, they found fungus inside furnace and showed us, so the price went way up high from $89 (only air duct cleanning) to $450 (which includes added fungus removal service using chemicals).

I'd like to get some advice from here: is it reasonable price for using chemical to remove fungus (not sure what chemical though), and is it a safe way, and is there any other method that can get rid of fungus? our house is about 3000 sf.

Thank you in advance!