There are alot of different car hire companies in the Costa Blanca but I have found they are all very differently priced. I book online with www.carjet.com 

They are very competitive the service and quality of their cars are excellent and you don't have to pay when you book online. You can reserve a car and pay when you get there if you wish. They also do not charge you if you cancel your booking just make sure you let them know so they can give the car to someone else.

When you get to the airport you will find that the company you need to collect your car from is Centauro. They are very helpful and they will explain exactly where your car is for pick up.

Prices start from around £60 a week for a small car but obviously you pay more the bigger the car you have.