Cheapest way-Take the RER and transfer to the Metro, the vast majority of Paris Metro, especially in a major tourist area like Musee de Louvre will have escalators. Its just like taking the Subway to the airport from Manhattan, it can get exhausting, but it saves a lot of money.
Way that I would not recommend-Air France bus, tried it once with my family. Dropped us off "near" our hotel, needed to take a cab to the hotel for about 10 euros to avoid dragging our luggage several blocks. Its not worth the money when everything is added in.
Easiest way-Take a cab. You can soak in some nice scenery on your way into Paris and relax. I don't remember how much it cost when I took one to CDG
from my hotel near La Defense, but I believe it is a flat rate.