One of the Air France busses (€10) goes to Etoile, which is at the 'high' end of the Champs-Elysées. The Marriott is a 5-10 minute walk from there.
Taxi is in the €35-45 range. Worth it if you have suitcases and are a party of 2 or 3.
RER + subway service (€7.60) will take you to either the Concorde or Etoile subway station (the Marriott is between the two), after a transfer.
I usually take the AF bus if it goes close enough to my destination.