I now know the source of the 11.40€ RER ticket from CDG into Paris. This is the price for paying in advance for a ticket (normally 10.30€) which makes absolutely no sense as you will still need to pass by the kiosks to have the prepaid ticket issued. Just purchase tickets at the kiosk and save time. Further, the 11.40€ would not take you beyond Paris, to La Croix-de-Berny for example. For that you would need to purchase a special point-to-point ticket costing a bit more than 10.30€.
A further point, resolving never to take another taxi from CDG because you were scammed by something other than a taxi does´t seem logical. You never really took a taxi in the first place and there are countless signs at baggage claim and elsewhere warning about these scammers. I have always had reliable service from taxis at CDG.
Taxis from Gare du Nord offer special problems and are a separate issue.