Hotel de la Paix Montparnasse hotel Paris

Best and cheapest. comfort means a lot.

Quality, Service, Price. Pick any two.

I would take the RER B to Denfert-Rochereau then take either the No 4 or 6 Metro one stop to Raspail, then walk about 300 feet. Or take the No 68 bus from D-F. Bus transfer will cost you another 2 EUR, but the Metro transfer is free.

And the hotel is quite a ways, relatively speaking, from Gare Montparnasse, at least 2 Metro stops.

Or you can take "Le Bus Direct" from the airport to Gare de Montparnasse and then walk or take the metro to the hotel (or a taxi). More expensive than the RER, but more comfortable, and still not as much as a taxi (55 euros). It's 17 euros one way, 30 euros if you buy a RT ticket.

You can check out the service here:

http://en.lebusdirect.com/lignes-horaires/ligne-cdg-aeroport-paris-gare-montparnasse.html

It's a bus that only goes between the airport and two stops, with storage space for your big bag underneath and space for carry-ons in the front, with comfortable seats and a/c. It can be slow, with the traffic into town, but at least you are comfortable and secure. Many a time I've snoozed on the way into town. You don't have to worry about your security in the way you do on the RER. But if you're a couple of people, at 34 euros for two, you're getting close to the price of a taxi direct from the airport (where you really don't have to worry about anything). So I guess it's a little bit between the very cheapest (RER and metro/bus) and most comfortable (taxi/uber direct to your hotel).

If there is only one of you and you have more than one suitcase, I would recommend Le Bus Direct.
If there are more of you and you have more than one suitcase apiece, I would recommend taking a taxi all the way if comfort is as important as you say it is. Four passengers and their luggage travel for the same flat fee to the airport.

In addition to walking to your hotel, you'll have to contend with turnstyles and a lot of stairs, if you take the Metro. So, this would be last on my list of options, especially if you are coming off an international flight.

I'd walk from the Le Bus Direct stop. The bus drops you off on rue du Commandant Rene Mouchotte, which is about a 15 minute walk to the hotel. By the time you walked from the bus stop to the metro station, got your ticket, dealt with turnstiles, etc. you would be just about at the hotel. Of course, you have to have manageable luggage for this to be a good option. If you do not have manageable luggage, take a taxi.

does anyone know about what the regular taxi from the line fare would be to the hotel de la Paix Montparnasse hotel?

55 EUR to Left Bank destinations.
http://www.parisaeroport.fr/docs/default-source/passager-fichiers/acces/forfaits-taxis-cdg.pdf?sfvrsn=2

The RER/Metro combo is 10 EUR per person.
The Bus Direct is 17 EUR per person. Either add another 1.90 EUR for the Metro transfer or walk 1/2 mile.