There are only a few dozen beds inside the walls and everything else is mainland, so the decision is how much you will pay to be on the island when the day visitors are not.
Inside the walls — the village itself
A handful of small hotels on the Grande Rue and the lanes off it. You pay a lot for a small room and you carry your bags up cobbles and steps with no vehicle access, and in return you get the Mont in the evening and at first light, which is a genuinely different place from the one people queue through at midday.
La Caserne — the built base by the car park
The purpose-built cluster of hotels, restaurants and the main car park on the mainland, where the free Passeur shuttle starts. It is functional rather than pretty and it exists because of the visitors, but it is the closest you can sleep with a car and it puts the first shuttle of the morning within walking distance.
Pontorson — the nearest railway town
The nearest town with a railway station, a short drive south of the bay, with the cheapest beds and ordinary restaurants that are not pricing for a captive audience. It is the sensible base if you are arriving by train or watching the budget, at the cost of needing a bus or a car to reach the Mont.
Practical tip — the village is free and open all night
Nobody checks a ticket to walk into the village; it is open around the clock and only the abbey is ticketed. Parking is also free between 18:30 and 03:00 from September to June, so an evening visit costs nothing but the drive and is the emptiest the Mont ever gets.