Mate! You're somewhat late with your questions!
What's your budget for a night in Paris?
Book your train ticket, Paris Montparnasse station to Bayonne. Try also to SJPDP to get the whole trip on one ticket. Get the ticket so it's available on your mobile phone.
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Best to get an early train, between 7am And 10am from Gare Montparnasse. Takes approx 4hrs to Bayonne and you want to arrive there to catch the train to SJPdP as services drop off as the afternoon progresses.
Too late arriving and there's only the expensive taxi to get to St Jean!
Try booking Paris Montparnasse to SJPdP to get both journeys on one booking.
If you don't organise a bed in SJPDP before arriving, head for the pilgrim's office first thing. They will help get you sorted as well as giving the best advice as to weather etc.
As far as a bed in Paris goes, look for something suitable close to Gare Montparnasse station due to your maybe looking at an early start.
Good traveling and Buen Camino pilgrim,
Regards,
Gerard