Loot, AKA: Item Elixir and Titan Shards
Loot that can be obtained from a specific City is scaled to the difficulty of the City, in comparison to your offensive power. (# of Skulls that you see a City as when you are viewing it in the Raid Orbs)
1 & 2 Skulls: Considered to be a weaker Defensive than your offensive power and will yield less Loot than a 3 Skull.
3 Skulls: Considered on Par with your offensive power and will yield a normal amount of Loot.
4 & 5 Skulls: Considered stronger than your offensive power and will have the ability to yield more Loot than a 3 Skull.
On average, you can receive 3 times the amount of loot for attacking a 5 Skull City in comparison to a 1 Skull City!