What's the simplest and cheapest way to get from central Nice, France to Monaco by public transportation?
Bus 100 (Nice - Monaco - Menton) is a very good option and costs a mere 1 €. It runs every 15 minutes mostly, and the trip takes 30-45 minutes. It takes an absolutely scenic route along the coast, passing through seaside towns such as Villefranche and Beaulieu.
(There's also express bus 100X which takes the motorway and costs 4 €; obviously not as good for sightseeing.)
(NB: when I took this bus in Nov 2012, the place of departure had temporarily changed due to construction work: instead of Station JC Bermond, it left from in front of Musee d'Histoire Naturelle, 60 Boulevard Risso. Ask around if needed, and you will surely find it.)
Arriving in Monaco, there are 6 stops to choose from (do get off at one of these as the bus continues towards Menton):
Heading from Monaco towards Nice, the route is slightly different; then the Monaco stops are:
The buses don't run very late: last one from Nice leaves at 20:00, and from Monaco at 20:22. First departures in the morning are about 6:00. You'll find details and full list of stops in bus 100 timetable.
Alternatively, take the train ("Nice Ville" to "Monaco Monte Carlo"). It is faster (22 mins) and arguably easier, but costs a little more at 3.60 €. There are about 2 regional trains per hour (daytime), according to SNCF. A good thing is that the trains run much later; last one leaves from Nice 22:25 and from Monaco 23:12 (on weekdays).
The above applies to regional trains (TER) which would probably be your primary choice on this route, though you could also catch an intercity/TGV.
I have just returned from Nice/Monte. Great trip and good advice given here.
As of Nov 31 2013 the bus costs €1.50 each way and starts just off Garibaldi Square, pay cash.
Also, the Nice day pass has changed.
It is a 24 hour pass that costs €5 and can be used on all trams and buses in Nice except buses 98 and 99.
What does this mean? The only bus that the pass covers that goes to and from the airport is the number 23 (catch it outside the Rail Station), takes 30-40 minutes to Terminal 1.
From there use the free airport shuttle bus to get to T2.
You ask, what about the 98 and 99 buses? Well they now cost €6 each way - the benefit is it is an express bus.