Looking for opinions on the best, fastest, and cheapest way to get between these two places. Ideally would like to do a day trip from Mont Saint Michel (not on the same day as visiting MSM) to Dinan and then back to MSM. I contacted one company offering private drivers. Cost for 3 adults is 260 Euro. Just wondering if there are cheaper options out there.
Public bus options take 3.5 hours one way with a transfer in Rennes. That works for a one-way transfer but infrequent schedules probably also work against the idea of a same-day round trip. Infrequent train connections can be a little faster but not simpler. Rick’s France 2020 book estimates 110 euros one way for a taxi, so that sounds in line with the price quote you already have. If you’re at all thinking of driving yourself in Brittany and Normandy, this would be another reason to do so.
Hiring a car would be cheaper. It’s about an hour’s drive each way. Dinard and Saint-Malo are also worth seeing.
Why don't you simply rent a car for the day. It will be cheaper and you'll have greater flexibility. I've driven from MSN to Cancale and back with no problems other than the speed trap. Believe it or not I talked my way out of the ticket by using the translation section in the back of the Rick Steves France guide. It was taking so long they let me go.