Car hire at Cluj Napoca airport is good, there are several offices of car hire companies, from hertz to local unknown companies that are normally cheaper. Cars start from around 30 euros a day including insurance. We had a group stay recently who rented a 9 seater for 70 euros a day and had it all booked and payed for before they arrived. You can book before hand or just turn up and you should be fine allowing you to haggle a bit if there are several cars sitting in the car park doing nothing.

Here are some web sites for car hire companies at Cluj Napoca Airport

Cluj Renta Car
Europ Car Rental
Argus Car Rentals