It’s not about the monetary expense, it’s about the logistical considerations of light sources. As
@reelo pointed out, carrying a torch or lantern occupies a hand, which can limit a character’s ability to wield weapons, manipulate spell components, or interact with the environment, unless they use action economy resources to put it down at the start of combat. More importantly though, it’s something you have to take into consideration when planning a delve into the dungeon. How many torches or vials of oil are you going to bring? Those take up inventory space/weight, and they can be a limiting factor on how long you can remain in the dungeon. Or would do, if the Wizard couldn’t just cast Light whenever they want. “Torches/lanterns go out” is also a standard dungeon complication, which doesn’t really matter if the players are relying on cantrips for light instead of torches or lanterns.