To get the best fare for Brussels-Paris, book ASAP (up to 90 days allowed) at www.thalys.com. A Smoove fare can be as low as 25.00 EUR. The standard 2nd class fare is 88.00 EUR.
The ride on the RER E from Magenta (attached to Gare du Nord) to Haussmann - Saint-Lazare (close by Gare Saint-Lazare) takes 4 minutes and the current fare is 1.60 EUR. (If there is a fare increase, it will take effect on 1 July.)
Actually, booking in advance (up to three months allowed) at www.tgv-europe.com can get you a Prem's fare as low as 10.00 EUR for Paris-Rouen. The standard 2nd class fare is 20.50 EUR. To keep the site in English and to avoid being bumped to the Rail Europe site which doesn't offer discount fares, choose Great Britain as your country of residence.
I recommend autoeurope.com. You can pick up a car at the Rouen-Rive Droite train station.
Unless you're going to spend some time in Rouen itself (like visiting the cathedral), you could take the train from Paris to Caen and pick up a car at the train station there.
The standard 2nd class fare for Paris-Caen is 31.20 EUR. The best Prem's fare is 23.00 EUR. Trains to Caen also depart from Gare Saint-Lazare.
Many people on this board highly recommend Battlebus for tours of the Normandy beaches. Battlebus requires that you spend the night before a tour in Bayeux or close by.