"See, the Ghostbusters have exactly the problem you'd expect them to have - they have virtually no money and still have to defend the city from a siege of ghosts. Even though the city direly needs them to solve its ghost problem, it's still going to charge them. Perhaps most ludicrously, there is an entire store dedicated to selling ghost catching supplies. The world is on the brink of ending, and they are still charging in a business-as-usual fashion...From there, you spend most of the game balancing cash flow — trying to catch enough ghosts so that you can actually afford to catch ghosts, while simultaneously trying to build up sufficient 'savings' to buy the equipment needed to survive Zuul's Tower, which, of course, you won't. Perhaps most tellingly, Ghostbusters is the only game I am aware of in which the world can end because you were too poor to buy gas."
"By the end of the game you are renowned everywhere as the Legendary Heroes, every surviving government and authority figure has rallied behind you, the fate of the world is obviously hanging in the balance, and out of nowhere random passers-by give you a pat on the back and heartfelt good luck wishes. However, shopkeepers won't even give you a discount, much less free supplies for the final battle with evil."
— The Grand List of Console Role Playing Game Clichés, "Adam Smith's Revenge"
"It doesn't matter how many times you save the day, a good merchant still won't give you a discount on weapons and armor."
— Talon, Breath of Death VII: The Beginning
Wakka: We gotta pay?! If we lose, you'll die too, buddy!
Rin: I have faith in your victory.
Wakka: Gee, thanks!
— Aboard the Fahrenheit, Final Fantasy X
Sorry, Uncle Donald. No family discounts!
— Huey, Dewie and Louie, Kingdom Hearts I
Tippi: But that's preposterous! You can't charge to teach the hero of the prophecy!
Bestovius: Why not? Even the great and splendiferous Bestovious has certain... expenditures. Why should the hero get the world delivered to him on a well-garnished platter? These heroes think they can get away with shaking people down for free skills! A mere 10,000 coins should be nothing for a hero of the universe.
Vincent: I have been ordered to assist you in your quest. I've brought potions with me, but, well...
Jonathan: Well? What?
Vincent: It's a matter of... economics.
"You greedy son of a bitch! We're out here trying to protect your sorry ass, and all you can do is think of ways you can rip us off?!''
— Ashley Williams, Mass Effect
Worker: I’m sorry, ma’am, but it seems like your credit card isn't working. Would you like to pay in cash?
Customer: What? That’s not possible. Try it again.
Worker: (after trying a few more times) Do you have a different card? This one might just be having problems. Or you could just pay in cash?
Customer: No! I don’t have cash. Just give me the items.
Worker: I can’t do that, ma’am.
Customer: Why not? I need them to stop the world from ending!
Worker: Sorry, I still can’t.
Customer: What sort of a monster are you? If the world ends, you’re to blame!
(the customer storms out)
Black Mage: Shouldn't I get some kinda discount on these spells? It's in your best interests that I get them, y'know. My use of those spells will stop you from getting killed by a dreadful monster. From outside our reality.
Magic Seller: These are the prices and like a fence or a short hedge, there is nothing you can do to get around that.
Legend of Zelda is an interesting case, along with every other game that has a (group of) destined hero(s) guided by prophesy to save the world. Strictly speaking, if they are the only people in the world capable of stopping the evil overlord from ushering in a A Thousand Years of Darkness then it makes sense to give them everything that is not nailed down or on fire that might be even remotely useful to them.
My own general rule is if the P Cs are on a quest to save [large population] from [horrifically bad fate] then they have the leeway to loot n' plunder them thar tombs.
— Commenter JayTee using this trope as justification for tomb robbing