It occurs to me that, once a megadungeon is discovered and starts being exploited by various adventurers, it's only natural that a sort of "capitalistic ecology" will begin to form around the dungeon environs.
For example, I once had a megadungeon placed out in a wilderness, which the player characters subsequently discovered and started hauling out the usual cartloads of treasure. Within a couple of weeks, other adventuring parties started showing up. A month later, a tavern started getting built. Soon the tavern added rooms, and a blacksmith showed up, then a gem-buyer, etc. You get the idea.
It occurs to me that a whole industry would, in the case of particularly large megadungeons, rise up. Sure, there would be the usual folks who sell $100 shovels and $10 torches, but there would be other, more ingenuous, sorts as well. The traders in treasure maps would descend first; not only selling but buying too-- the trade in information is invaluable. Hirelings, of course; need a cheap spear or sword? They're probably going to be readily available, but at a stake of a share of the loot. The usual run of alchemists ready to buy the odd stirge proboscis and such, but maybe the alchemists don't want to deal with the violent and dirty adventurers directly. Perhaps there's a whole class of middle-men who take the greasy grimy griffon guts and pass them along to the folks who turn them into the wonderful potions they keep finding in the dungeons.
And then there are the specialists, like Mr. Quint quoted above. Why wouldn't there be people who you would hire, on a one-off basis, to fight the tough baddie you just can't get past? The troll-hunter. The dragon-slayer. The purple worm tamer. They'll all have a reputation, and they'll insist they're in charge, and they'll all have a very high price for their services...
Guides, too. If there can be guides for the wilderness, who can take you through the forest and over the ridge to the next valley, then surely there are guides who will hire out to take you to the third level of the dungeon. Maybe they were the lantern bearer in the ill-fated expedition of the Knights of Holy Fire. They saw the way through, and managed to scrape out alive, and they'll show you, too... for a price.
I see a whole complex of NPCs ready to help, and take advantage of, player characters hitting the enormous dungeon complex "but a league east of the city". Might make things a lot more colorful, when it comes time to change in those gems, or when the party is flummoxed and some helpful chap happens along with a map that "is guaranteed to show you the quickest route to the fourth level of the dungeons!"