Marcus Crassus had his own firefighting division, whom he could deploy to properties which were under the threat of fire (common occurrence with the insulae) . He would then demand the property for dirt cheap and would refuse to put out the fire until it was in his hands. After that- well, it depends. If it costed less it would make sense to fix the place up. If it was unsalvageable he could easily build another house in its place and rent it out. Eventually though he made enough money off of it to fund his own armies (something he always boasted about).