The economy will be based on them.

1 Diamond → 3 Emeralds
14 Iron Ingots → 1 Emerald
3 Gold Ingots → 1 Emerald

Template:

• Offered Item: Cost (+ Secondary Cost) → [Amount]

Eggs to spawn animals and passive mobs when you can't get them in another way (like healing zombie villagers) - that's why some of them are expensive.

• Pig: 6 Emeralds → 1
  
• Sheep: 6 Emeralds → 1
  
• Cow: 8 Emeralds → 1
  
• Mooshroom: 18 Emeralds → 1
  
• Wolf: 8 Emeralds → 1
  
• Ocelot: 8 Emeralds → 1
  
• Villager: 27 Emeralds → 1
  

• Mob Head (Zombie, Skeleton, Creeper, Player): 1 Wither Skull + 2 Emeralds → 1
  
• Special Full Double Slabs (43:8 - Stone and 43:9 - Sandstone): 64 Stone or Sandstone Slabs + 1 Emerald → 32
• Sponge: 2 Emeralds → 1 \\

Most difficult part concerning the game balance, as the items are either very rare (the Stronghold variations of Stone Brick) or difficult to obtain and non-renewable, even if they aren't worth much (like the plants).

• Lily Pad: 6 Vines + 1 Emerald → 6
  
• Dead Bush: 6 Oak Saplings + 1 Emerald → 6
  
• Fern: 6 Tall Grass Blocks + 1 Emerald → 6
  
• Cobweb: 30 Strings + 1 Emerald → 6
  
• Saddle: 5 Leather + 1 Emerald → 1
  
• Chiseled Stone Brick: 5 Stone Bricks + 1 Emerald → 5
  
• Cracked Stone Brick: 5 Stone Bricks + 1 Emerald → 5
  
• Mossy Stone Brick: 5 Stone Bricks + 1 Emerald → 5
  
• Mossy Cobblestone: 6 Cobblestone + 1 Emerald → 6