The drop off fee France to Italy is hefty (Hertz charges approx USD 500, for example; you will find most rentals will be in that range). So expect to pay about USD 1200 for a Ford Focus size car, up to double that for a larger car. It might be cheaper to rent for 1 week in France, dropping at Geneva, and then renting another car from the Swiss side (so it is one way Switzerland to Italy).

Versaille is a good pickup location in Paris if you don't need a car in the city (and you do not!). Easy to get to the highway and out into the countryside without having to learn how to drive in Paris.