You could do this trip more cheaply as a self-drive activity, but we wanted a break from driving. Having a guided tour made for an easy and relaxing trip.
Our driver/guide (Marcia) was passionate about the canyon and full of lots of facts and first hand experience of living and working in the canyon, which is much more interesting than simple turning up at a view point to peer over the edge.
Not sure if it's unusual, but we were the only people on the trip, so it a was effectively a private tour. Pace was just right for us and we could spend as much time as we needed at each stopping point. Picnic lunch provided. Also received some good advice from Marcia about where to go to the following day.