Originally Posted by
MSPeconomist
IIRC one of the airport bus routes goes to Montparnasse. It will be a lot cheaper than a taxi and avoid the risk of a huge fare if traffic is bad but of course it won't save time compared to a taxi and might actually require a bit more time if there are other stops. It's also easier with luggage than the RER and metro combination, especially in rush hour.
No risk of huge fare-fares are fixed from both Paris airports.