It varies day by day and the customer cannot predict it.
If there is an expected shortage of cars at one location and surplus of cars at another, then the rates and fees charged to the customer can be lower for a one way rental from the surplus location to the shortage plagued location compared with return to the same locations where the rental began.
Other factors also influence rates, for example airports charge fees to car rental agencies which in turn pass the fees to the customers. There are different permutations of taxes, fees, competitive discounting, high demand such as Christmas or Daytona Week, and surplus/shortage issues that all affect the final cost to the customer.