Instead of renting, you can also try to buy a Virgin Mobile prepaid phone. It costs 79€ and is definitely cheaper than renting.

A basic Nokia phone with 10€ airtime costs 79€.

You need to go to Virgin Megastore on the Champs Elysees or at any Phone House store (several around Paris).

Alternatively a Virgin Mobile SIM card cost 25€ with 10 min airtime.

They use the Orange network in France, so coverage should not be an issue

Hope this helps