There’s free entry to Mont-Saint-Michel and the abbey on the first Sunday of the month (November to March).
I went on the first Sunday in February 2019 for what turned out to be a beautifully sunny day. It was a little chilly but there were glorious blue skies out there and the sun was shining.
The full price is 10€ for an adult so you can make a good saving. There is a charge for car parking and you can catch the free shuttle bus to the island. You can read all about the island, the village, the abbey and the surrounding area in the comprehensive guide to Mont-Saint-Michel. It includes photos, videos, hints and tips that will help you to make the most of your time there. The free entry to the abbey is a great opportunity to discover this amazing place.