Does the hotel provide shuttle service to the Nice airport?
When we did the tour several years ago they did not, but things may have changed since then. We shared a cab with two other tour members and don't remember the fare being all that bad. The airport is a good way from the town. Can't remember if there was a train to it or not.
The airport is about 20 minutes by cab or 30 minutes by bus from the hotel. They are building a light rail to the airport, but it's not ready yet (depending on when your tour is, it may be).
Very few hotels in Europe provide shuttle service to the airport. But it sounds like this is the end of a tour, so your tour guide will be able to provide assistance for getting to the airport. As said above, you won't be the only one going, so people often split taxis, and guides help arrange this. Or, since you won't be jet lagged and will be familiar with public transportation, you may want to get to the airport that way.
The new tram line to the airport is supposed to be open by the end of 2019. This will be very convenient to the hotel.
Your tour guide will facilitate ride sharing options with fellow tour members that are leaving at about the same time if you are interested. I was able to share a town car with 5 other tour members out to the airport at the end of a different Rick Steves Tour and it was by far both the least expensive and most convenient option.
Taxi fare will run you about 30 to 35E, an Uber a bit less (15 to 20E). You can also prebook a van or car, which can be shared. There is an airport bus line, 98, that runs along the Promenade (6E pp). However, the nearest stop (Gustav) is 0.5 km away, a haul with luggage. The tram line from near Place Massena to the Airport is currently set to open in June 2019 (this has slipped many times, but at least now part of the line is running, just not all the way)