The walk and other alternatives described in the previous post are both unnecessary.
Instead simply change "platforms" in Gare Routiere and take the 200 bus sign posted Cannes. This will take you to the airport for 1 euro 30.
The timetable is at:
http://www.cg06.fr/transport/transports-tam-lignes.html
To answer your question however, although the above is cheaper - it certainly isn't better. The advice is obvious - please just pay the 15 Euros and take the 110 coach! It's great value! The timetable is also at the link above. Also if you have any luggage the 110 coach opens the luggage lockers underneath to take your luggage. It's first stop after Monaco is the Airport. The local buses however will not welcome any luggage -and I promise you that you will not be a popular bunny if you take on cases or anything larger than a shopping bag.