Easiest way to travel from Charles De Gaulle Airport to Gare St. Lazare

Since you say we take a taxi from the official stand at CDG. They have a fixed price I think 55 euros you can split between you.

St Lazare is on the Right Bank, so it is only 50 EUR.
If you want to be more thrifty and adventurous, take the RER B to Gare du Nord and transfer to the RER E to Gare St Lazare. About 10 EUR each.

You could also take the train with connections (one part of the service is the TGV) from CDG to Caen or Bayeux or another destination in Normandy.
You have to research this http://www.sncf.com/

Easiest is a taxi for 50€ and is what I would do. Will it be the fastest? Who knows. Surface traffic is subject, in my experience, to more delays than mass transit. Taking mass transit (the RER and connections) requires more walking (about 15 minutes not including the walk to the train from the baggage claim) with luggage than I ever want to do after a 13 hour flight. You could take the Roissy Bus but then you have a tiny bit less walking and it is direct from CDG to Paris and you do not have to manage your luggage during the trip like you have to do on the train.

Edit: Generally, trains to Normandy from CDG take much longer than the time it takes to get to Gare St. Lazare and take a direct train there. That being said, if you are taking a train from Gare St. Lazare, allow 4 hours between the ETA of your flight and the departure time of your train.

I did this CDG to Gare du Nord to Gare St. Lazare this past Aug. on the Metro. I've been a Paris Metro user for years with no problems, but I've got to say that Gare du Nord is HUGE and the signage to find the train to St. Lazare was poor. I had to ask directions.
So, if you're on a strict budget, it can be done, but be willing to ask questions of strangers. If you are willing to pay a bit more, you will be a lot less stressed.

If you reserve a car service in advance, all you have to do is walk with the crowds to the baggage claim area and look for someone holding a sign with your name on it.

There are cheaper ways to Gare St. Lazare, but this way requires the least thinking.

Thanks everyone! Sounds like a taxi is the way to go!