This is just a little bit of foreshadowing. Anyone in a book who spends as much time on their collection as Dan does is going to have to give up some, or all, of it someday.
"Each of you must sacrifice your share of the inheritance to participate." (2.106)
The hunt begins with a sacrifice. While it's not a big deal to many of the Cahills, who are multi-millionaires, it could change the lives of Amy and Dan. It shows how determined they are when they make this sacrifice anyway.
On Amy's orders, Dan was only supposed to pack one bag. (6.78)
Ah, we totally saw this coming. Dan has to leave his entire collection behind to go on the hunt. Not only that, but he offers to sell it. This is a major sacrifice that shows he puts his family, Amy, above material goods.
[Dan] wasn't an expert at jewelry prices, but he figured that [jade dragon] necklace was one of the most expensive pieces in the box. If [Amy] kept it, they wouldn't nearly get as much. (6.98)
And Amy does keep it. The jade dragon is so unique, it'll probably end up being a critical part in the plot, but unless she's psychic, Amy doesn't know this. Instead it looks like she's not sacrificing as much as Dan does.
[Dan] wanted to go on this clue hunt more than he wanted his collection. (6.105)
Dan's sacrifice is huge. He doesn't just leave his collection behind, he decides to sell it to finance their trip. If they win, Amy's going to owe him enough baseball cards to wrap around the globe.
If [Saladin] got hurt, Dan would never forgive himself. (9.64)
Saladin the cat has become a beloved member of Amy and Dan's family, but they give him up (temporarily) to keep him from getting hurt. The things we do for our pets.
Dan gave the backpack another tug, but it wouldn't budge. He looked at the train, and Amy saw he was crying. She didn't understand why. (16.125)
Dan's falling into a stereotypically male trap of not talking about his feelings. Or maybe it's just because he's eleven. In any case, he doesn't actually talk to Amy about their parents. Maybe if he did share his memories, losing the photo in his backpack wouldn't be as big of a sacrifice.
Maybe [the photo] had been ripped to shreds, or maybe [Dan's] parents would be smiling in the darkness forever with no company but the rats. (17.2)
The photo of his parents is the one sentimental thing he brings on his adventure, and now Dan's lost that. At this point, he's pretty much given up everything he holds dear for this clue hunt.
[Amy] held out the glowing green vial. Ian took it and she snatched the antidote out of his hand. (18.76)
This glowing green vial is a valuable piece of the 39 clues puzzle, but Amy has to give it up to save her brother. Even though the hunt is a higher priority to Amy than money, this shows that her brother is number one in her mind. We doubt Ian and Natalie Kabra would do the same for one another.
"No, thank you," Amy said. "We'll get by just fine." (19.55)
Mr. McIntyre offers Amy one last chance to sell Grace's jade dragon necklace. She's not willing to part with it, even though she and Dan basically have no money and no means to continue the trip. Why does she insist on hanging onto it?