Nothing says "treasure" like a gold coin, but a real Spanish doubloon will set you back several grand. Here's a personalized gold coin for 17 cents. Your local coin store will sell 10-12 unsorted foreign coins for a dollar. Pick out the big gold and copper coins with extra credit for two tone currency, odd edges, and center holes. Now take your pirate's stash home and make every coin collector groan by removing the tarnish with silver polish. They clean up real nice. Place the shiny coins in half dollar coin mounts and glue them shut with superglue. Now print heads and tails images that fit the mounts and glue them on (use a mount with the plastic removed as a template for cutting out the circles). Finally, make your freshly minted cache coin water-resistant by puting it in a crafter's mini-ziplock. Click here for a 1.5M pdf design: http://renzotobias.googlepages.com/cachecoin.pdf
Foreign Coin: $0.10
Coin Mount: $0.07
Seeing a child's face light up when they find a gold coin: Priceless.