Avignon to Mont St Michel

Unless you want the car at both ends (or it throws you into a couple of expensive short-term rentals), driving isn't te way to do it. You're looking at a mean nine-hour drive if you make only a couple of real quick stops. Tolls will probably cost more than the gas, maybe a couple hundred bucks combined. The best you could hope for by train is a change in Paris which would entail using two stations on opposite sides of town (Lyon and St Lazare). I'd guess it'd take about half the time. I don't know how to figure prices, but it'd be cheaper than flying. To fly, it'd probably be Air France from Marseille to Caen with a change in Lyon. You'd probably be able to pull it off in three hours for about four hundred bucks.

FYI, the train will only take you to one of the nearby towns; you'll then need to catch a bus to MSM. Check the train routings on Deutsche Bahn website for specific durations (although you won't be able to purchase through them). If you decide to look into flights, check skyscanner to see if any of the low cost airlines would be of use. And FYI, the one time I used AirFrance for an internal flight (Lourdes, in the far SW of France, to Paris), it was much cheaper to buy a round trip ticket and only use the outbound segment than to buy a one way ticket.

Disregard most of my previous post. I was thinking Caen for some reason. The driving time would be about an hour longer, the gas and tolls maybe ten percent more. The second Paris train station could still be St Lazare, but more likely Montparnasse, depending on the departure time from Avignon. Pontorson would be as close to MSM as you can get - - but buses will scoot you the last five miles in short order. I'm still thinking Air France, but the closest I've seen them is either Caen or Rennes. The rail trek from Rennes is probably about an hour. From Caen it'd be about two, but departures are infrequent so there'd probably be a connection problem.

Take the TGV from Avignon to Rennes. Then take the regional "TER" train from Rennes to Pontorson. The Pontorson train station is 10 Km from Mt. St. Michel, there are shuttle buses. Journey takes 8 1/2 hours, not counting the shuttle bus ride. You didn't say when you were traveling. I arbitrarily ran dates in mid-April, and the tgv-europe website gives a price range of 51-67 euros for the total Avignon>Pontorson trip, depending on which day of the week you travel. You won't beat that flying or driving. According to the tgv-europe website, the only train change you have to make is in Rennes.