In Harry's first year on the train to Hogwarts, Ron doesn't have the money to buy something when the woman with the cart comes.
Ron’s ears went pink again and he muttered that he’d brought sandwiches.
Harry bought, among other things, some Chocolate Frogs.
“What are these?” Harry asked Ron, holding up a pack of Chocolate Frogs.
“Oh, of course, you wouldn’t know — Chocolate Frogs have cards inside them, you know, to collect — famous witches and wizards. I’ve got about five hundred, but I haven’t got Agrippa or Ptolemy.”
Harry Potter and the Philosopher's Stone, chapter 6: "The Journey from Platform Nine and Three-Quarters"
How did Ron get five hundred cards? Where did he get the money for them? Sweets with cards probably cost more than sweets without cards, and Ron doesn't have the money to buy any sweets.