There are probably over a doz of stations among Avis and Hertz inside Paris to pick up / return the car.
You need to carefully go over the locations to find the ones that are easy to leave the city and to return, assume they dont want to do it at CDG.
Railway stations in secondary cities often are good locations too. They would incur surcharge, just like at the airport.
Plus you said one-way rental is "very expensive". Not always. You do not know UNTIL you start the booking process. It very much depends on where there are lots of needs for cars to move from one place to another place. If the traffic is there, the one-way fee often is very low, or sometimes non-existent, or even at much lower rate than a return to same location rental. Your assumption on expensive one way fee is not always correct, even in Europe.
