There are 2 island unlocks currently available. The nether and the end. You cannot get to either (even by use of a portal) until they are unlocked. Blocks associated with each are also unable to be placed on an island without the associated unlock.
Note: Unlocks only provide extra buiding area in their specified world. No spawners or valuables are included in the unlocked space.
Cost: $100,000
/is unlock Nether
Cost: $500,000
/is unlock End