The problem with fireproofing one building in a fire-prone area is that neighboring buildings that aren't fireproof will build, and they will release toxic substances that make your fireproof building toxic.
Right; and houses release those toxic fumes when they're on fire. If poisoning other structures with your house fire was an actionable tort, insurance would incentivize owners (and builders) to make houses that were less likely to catch on fire and poison others, and/or find less toxic building materials and furnishings.
Fireproof to what level? Pretty much anything will burn if you get it hot enough. The cost will depend on what level of fire resistance you want to target.
