Symbolism, Imagery, Allegory
Anytime a character in a novel dreams about something, your symbol-alert alarm bells should go off. If a character in The Alchemist dreams about something, you should probably call up the fire department and the police station, because you're in serious danger from getting hit on the head with some symbols. Specifically, treasure.
The treasure shows up early on, in a dream that Santiago has about a child transporting him to the pyramids and telling him that he'll find a hidden treasure there. And the treasure does turn out to be pretty awesome, "a chest of Spanish gold coins. There were also precious stones, gold masks adorned with red and white feathers, and stone statues embedded with jewels" (Epilogue.7).
But gold coins and feathery masks aside, we get the idea that the material riches, awesome as they are, aren't all the treasure symbolizes. In fact, just before Santiago digs it up he wonders why he had to travel all the way to Africa if the treasure was in Spain all along. He gets his answer on the wind. "'If I had told you, you wouldn't have seen the Pyramids. They're beautiful, aren't they?'" (Epilogue.6)
In other words, we're getting a journey-not-a-destination vibe here, where the treasure isn't just the end of the rainbow; it's also the beauty of the rainbow itself. See, the treasure isn't just about Santiago getting rich; it's a symbol for the wisdom he has gained along his quest.
Priceless.