I built a fire room into a house addition for a good friend back in the 80's. The door and frame with bank vault type lock cost about $1800 and the poured lightweight concrete walls and roof cost about $8K, but he had over $200K in firearms in storage. This room was hidden in a walk in closet in the master suite with the door behind a sliding panel. I have built file storage into school offices for permanent records using concrete filled 12" block and poured light weight concrete roofs and the expense was not that much. If I only needed this type of storage now.