Travel from Paris to Normandy

The trains take about an hour and a half from G. Lazare to Caen and run every hour or so. I don't know what they cost. You can drive from Paris to Caen in two and a half hours, burning about twenty-five bucks of gas and about the same amount in tolls. You're using the car on a day for which you're going to pay anyway. You can leave when you're ready. The drawback is that you pass dangerously close to Rouen, making it very tempting to stop for a couple of hours. I'd rent from the spot closest to my hotel since the escape from the city is simple (head west until you hit the peripherique, turn left or right as necessary to hit the N13, then follow the signs toward Rouen until you sort yourself out). Failing that, the escape from Paris Nord is really simple and, early in the morning, traffic on the beltway headed counter-clockwise should be rather light. Car rentals are also readily available at the Caen station. You can return the car at the airport, not near it.

IF we arrive at CDG must we go into Paris to get a train to Normandy, or is the train station connected to the airport for a more efficient use of time/$$? It seems if we have to taxi/or whatever to the train station, pay for the train to Caen........rent the car........etc., it may be more efficient ant even less expensive to just rent a car at CDG and do it that way.?

Get the car at the airport and you can be in Caen much sooner than if you went into town, caught the train, etc.
There's no train from Roissy to Caen.

If you're going straight from CDG to Normandy, don't take a train somewhere to rent a car. Just pick the car up at the airport. There will be a small fee to pick up at the airport, but it's cheaper and easier than trying to connect to a train to Caen.