It's a good question. I don't know much about the crawling of dungeons, tombs, or what have you, but unless they're crawled more than once, they would be left fallow, provided that everything has been killed in them. So, what happens to them after that? Seems like a waste of good space that could be repurposed for other uses. So with that thought in mind, what if the players were given the option of taking possession of the dungeon or whatever it is either for their own purposes or handing it over to the nearby settlement or authority for their use.
But what to use it for? Since dungeons and tombs are underground, they would be cool enough, temperature wise, to be usable for storing food, which would make it a profitable venture to rent it out to local farmers, vendors, and taverns. Cheap housing also comes to mind. It wouldn't be very desirable to live in a dungeon or a tomb, but it would sure beat not having anywhere to live. You could also sell it to any interested wizards.
I'd hate to be the guys hired to clean one of those places, though. Can you imagine having to haul out of the goblins, trolls, orcs, zombies, skeletons, etc. corpses. And the dusting. My god, the dusting.