Hi Janeh,
Hiring in car in Spain is quite expensive. Last year after my Camino I needed to return to Madrid but there were no places available on trains or buses (at least for many hours). I hired a car to drive back. I was lucky enough to find two Spanish girls who wanted to return to Madrid as well and we shared the cost - it was €160. By dividing it, it came out to roughly the same price as the train ticket so it was ok. But the expensive part is picking up in one ciy and returning in another.
I had tried to do the same in Madrid to go to Valencia two years before but the cost was prohibitive compared to the price of the train. For this reason I don't have much experience to share with you. I can tell you that the rental car companies are located just at the side of the train station in Santiago de Compostela and I believe there are 4 of them there. Budget, Hertz, Eurocar(?) and possibly Avis.
I would suggest that you make the reservation well in advance from home so your car is ready to collect when you arrive. In terms of a deal, I don't think I can help, sorry.
You walk past the airport on your way to Santiago...it is before Monte de Gozo - I would guess approximately 10k outside the city.
Buen camino,