What is the cheapest way to get from Cannes to Montecarlo?

The best way (and also the cheapest) is to take the train. You do not need any reservations and the trains generally run about twice an hour. Just walk from the tender pier (or take a taxi) to the Cannes train station (its about 2/3 or a mile and you get to walk through the best part of town). The train will take a little over an hour.