The village of Mont-Saint-Michel is free and open day and night, but reaching it and seeing the abbey are paid in several different ways, sold by different operators: coach and minivan day tours from Paris, smaller tours from Bayeux and the D-Day coast, the TGV to Rennes with a direct coach, private driver-guides, the abbey ticket itself, and guided barefoot walks across the bay from Genêts or from the foot of the Mont.
Every route ends at the mainland car parks, where a free shuttle crosses the bay to the rock. Which way in is right depends on where you are staying, how much planning you want to do, and if you want to walk the sands with a guide. These pages compare them plainly.