The Return relocation is used when you need to return to you own Kingdom duing a Kingdoms Battle (KvK) or Kingdoms Battle: Revenge (KdR), and for the first two KvK's this item cost 100 Gold in the Item Store. With the 3rd KvK (June 17, 2016) the cost was raised to 3000 Gold. The current cost (Jan 13, 2017 are 1500 Gc). In the clan store this costs 17500 Loyalty Points.
You may not return to your own kingdom if you have troops outside your town or if you have inforcement troops in your town.
To get to the other Kingdom in the first place, you need a Great Relocation (see more info here).
If you are still in the enemy Kingdom when the KvK (or KdR) ends, your castle will be returned without any cost. Any reinforcement troops will be returned to their own castle (instantly, without delay).