The 50 euro taxi estimate would be one way. It would probably be a bit less.

The Eiffel Tower would be about 2 km from the Arc de Triomphe. that's only a bit over a mile

Last month we walked from the Arc de Triomphe via Avenue d'lena to the Eiffel Tower. Not far at all, maybe a mile. Depending on the time you might even be able to hop on the Batobus which stops at all the major sites and cruise the Seine until you need to return to CDG. Of course, lunch will be necessary too! Have fun!

Doesn't quite go to Eiffel Tower but for 2€'s easyBus is worth a look, it's supposed to be available from April 2015