Symbolism, Imagery, Allegory
In the world of Don Quixote, money is something that certain characters can take for granted and other characters can't.
More specifically, rich characters tend to not care about money, while working-class characters do care about it. Don Quixote, for example, is an aging man who has so much money that he gets bored with all the free time this allows him. This is why he turns to chivalry books in the first place: "You must know then, that when our gentleman had nothing to do (which was almost all the year round), he passed his time in reading books of knight-errantry" (1.1.1.2).
Don Quixote becomes so obsessed with his books that he's willing to take big financial hits to keep buying and reading them: "[H]e grew so strangely besotted with those amusements, that he sold many acres of arable land to purchase books of that kind" (1.1.1.2). Having that much money allows Don Quixote to indulge all of his whims. Yeah, it turns out that a lot of people get a kick out of the Don's antics, but these antics also cause a lot of trouble to people who can't really afford the trouble.
Now, Don Quixote's lax attitude toward money is the exact opposite of Sancho Panza's. Sancho spends nearly all of his time thinking about how to get three things out of life: sleep, food, and money. When he finds a young man's wallet in the mountains, for example, Sancho is more than willing to lie to keep the money for himself, because "the gold he had found, which was above a hundred ducats, had but whetted his greedy appetite, and made him wild for more" (1.3.9.8).
Sancho goes on to spend all of this money. Further, many of Sancho's conversations with Don Quixote in Part 2 of the book center on the question of how much money the Don should pay Sancho for being his squire. No doubt about it, Sancho is obsessed with money because he doesn't have very much, and Don Quixote is careless with money because he can afford to be.
Money causes all kind of problems for other characters in the novel, too. Doña Rodriguez is a noblewoman forced into servitude, for example, and the captain is afraid to return to his family without any money. In this world, money more or less equals status, though it's still better to be a poor nobleman than poor peasant. Have things changed?