When you plan to take public transportation from the airport it always depends on your exact destination in the city and on number of passengers.
Taxi offers flat rate of 50 € from CDG to the address on right bank, or 55 € for left bank - door to door service for up to 4.
It will be for a single person, so the most cost effective would be either Roissybus or Le Bus Direct. From the previous post, the fare for the Roissybus is 12 Euros and Le Bus Direct is 17 Euros.
Do I need the EXACT fare for the Roissybus and/or Le Bus Direct to purchase the ticket or will the driver make change?
Le Bus direct has a ticket machine right by the bus stop at CDG, so you don’t have to pay the driver anything if you don’t want to.
You can also buy the Le Bus tickets online which is probably the safest option to avoid any possible hassles with getting change or card acceptance.
The most cost effective is RER + métro for €10.30. There is a cheaper solution but it takes for ever.