Since you are arriving rather late in the evening a taxi would surely be your best option. I have no first hand experience with the taxis from CDG to Paris, but I´ve seen fares around 50 euro mentioned.

You could also take the Roissy bus:Se details here http://tinyurl.com/32ucu
- and then make it by metro, the rest of the way. Nearest metro to you hotel is St Placide, but Rennes is also close by as well as the large Montparnasse station .

Third option are the RER trains from the airport. You would get off at Denfert Rochereau, and then change to the metro line 4, that goes by St Placide. Details here: http://tinyurl.com/2yzww. Note that you can buy a combined RER and metro ticket for 7.75 euro.