7 min in... helicopter (Heli Air Monaco). 73 € one way + airport tax. They operate every 20 or 30 min from Nice airport. They have a complimentary pick up / drop off shuttle service in Monaco. I took them once 2 years ago and found their service excellent (but the flight was too short

). Needless to comment on the fantastic view over St Jean-Cap-Ferrat, Beaulieu and Cap d'Ail.
Bus is 23 € one way, every hour between 09:00 AM and 09:15 PM and it takes 45 min (can take much longer depending on the traffic jam on the highway and especially entering in Monaco).
I don't remember the price for a cab, certainly somewhere between 23 € (bus) and 73 € (heli). If it's over 50 €, the helicopter is a no brainer IMHO.