Can you recommend a reasonably priced door to door transfer service between the airport in Nice and Monaco?
You asked about a transfer service rather than a taxi, but here is what I found on the official Nice airport site concerning taxis. Expect to pay €69-74 for daytime rides and €79-90 for evening rides. I don't know anything about transfer services. If you want to save some money, take Express bus #110 directly from the airport to Monaco. Buses run every 30 minutes and the fare is €20. (Information is from the airport site.)