2h. 20 mins to Paris, 1 hr from Paris to LM. Allow a good hour to get from Gare du Nord to Montparnasse, just to be safe. Mr T does this a lot. The Lille way is much easier but only one train per day connects well. My Ma's doing it next week the Lille way so I'll be able to give you a full report when I get out of hospital some time in the New Year. I'm not sure about the cost as we were both staff so don't pay the full whack.