Sorry I don't. I am French and I always take public transportation from CDG but I would definitely not trust any company that would charge differently if you pay cash or credit card.
We recently did the transfer and had to pay 5 euros more to pay the driver on a card rather than cash. I assume it is to cover the percentage that the credit card companies charge the business. It was more convenient for us to use a card than to go and get cash; they were transparent about the additional charge up front and it was up to us which method of payment we used, so I was OK with it.