My friends in Paris take the RER. I usually arrive late at night, so I take a taxi. It's always 50 Euro, give or take, to the Eiffel area where I stay.

It should be noted that there is no RER sign at the Cluny entrance to the Metro, but once inside, it is clearly marked how to walk the corridor to it at St. Michel. You will encounter so much less "traffic" in there than on the sidewalks outside and within the St. Michel entrance which is often a total "zoo".