How much should i pay UE4 developers?

i will be paying developers to put together pre made assets , develop cities from references , characters etc. how much should i pay them? im new to all this and i dont know how to use UE4 but i have access to all the assets in the store and know what i want . i will be paying for one job to get done at a time .

.Building my cities & landscapes

  1. configuring AI , enemies

  2. Physics etc.

It will be a open world , GTA style game .