Bear in mind that you are going to have to pay the company to send someone to Spain to drive the car home. If they get a rental for the return leg, it's a bonus for them, not something they plan on.
Just for fun I checked the Hertz rate for a car picked up at Figueras. This is the Hertz Rental within Spain which is closest to Perpignan (31 miles) on the route to Granada.
Figueras to Granada (640 miles/1030 km) _= �_70.52 Perpignan to Granada (671 miles/1080 km) = �781.85
...quite a difference in price for the sake of crossing a border.