Many isekais show MC making crazy deals that make them rich and next chapter they're struggling to pay generic weapons, what's up with that? If the author wants to make them wealthy just be consistent latter. . . Yeah I know he's buying crystals which are very expensive but still, all the mirrors, jewels, amber and dungeon loot should net him a comfortable life
Truth be told dungeon economy on this manga seems a loosing enterprise if MC's having a hard time advancing the lower floors and his equipment is OP, like weaker adventurers could never really live out of this if they need more people for the same loot and improving gear's not worth the returns.
Nevertheless, economy aside, world building's quite good.