U.S. to Paris to Caen & ...

1.) Trans-Atlantic flight prices depend on your point of departure; your route (making connections instead of a non-stop flight may save a little cash at the expense of more travel time) and the time of year. Generally the top prices are June-late September. You can diligently search the many travel sites offering tickets such as Kayak, Orbitz, Cheapo, Hipmunk and many others, and then compare the rates to what is offered by the airlines themselves. For a general overview, www.itasoftware.com (click on "airline search") shows the information it provides to many but not all airlines, although it does not sell tickets (and does not cover a lot of the so-called budget airlines, which may show up on www.skyscanner.com) 2.) Yes you can take a train to Caen where there are some car rentals near the station. You will have to use Paris commuter trains from the airport to reach the inter-city departure station.
Train info: www.seat61.com&#59; http://www.ratp.fr/en/&#59; www.parisbytrain.com&#59; http://en.voyages-sncf.com/en/

I was just in Caen. There are four car rental offices right across the street from the train station: Europcar, Sixt, Hertz, and Avis. If you are going to Germany, Sixt may have the best deal for you.

What you don't want to do is pick up the car in France and drop it in Germany unless you have a monstrous wallet. Figure an extra three hundred bucks, minimum, if you do this. One way to do it would be to get the car at Roissy/Orly, motor up to Caen (you can drive faster than you can take the trains and it's easy), do whatever you want there, then turn the car in at Paris Nord (easy to find coming back from Normandy). From Nord there's a train to Cologne that will make if faster than you can drive. If you're coming home from France, just drive the whole way since driving from Caen straight to Cologne is faster than driving back down to Paris and then riding the train, plus the scenery is pretty good cutting across Picardie.