Le Relais Du Roy is a very nice place to stay when visiting Mont St Michel. Although it is very basic, akin to an older Motel 6 in the US, we found the room to be clean, the receptionists exceptionally friendly, and the breakfast buffet bountiful and delicious. The location can't be beat, as it's only a 30 minute walk to Mont St Michel, or under 10 minutes by free shuttle, which were running until 11 pm in May. There are 3 restaurants in La Caserne for dinner, all within walking distance of the hotel, so it's imperative to make a reservation if you don't want to dine on Mont St Michel. (We chose to dine at Le Pre Sale and the food was delicious.)
I would highly recommend staying at Le Relais du Roy when visiting Mont St Michel.