Maintenance on my 177 RG has been pretty close to what I saw on my 56 172. I pay a little more ($300) for my owner assisted annual but my IA really knows Cardinal RG's, is picky as all get out, and enjoys teaching me how to work on my plane. Knowing my plane, being picky as all get out, and enjoying teaching me to work on my plane were key attributes I was looking for in an IA so m happy with him.
The bigest cost differences I've had were replacing a vacuum pump (my 56 had venturies) and this year my Garmin 430 had to go back for repair. I've had no gear related issues but we check it closely at each inspection and lube it per the manual. I did send my dual mag in for the 500 hour inspection last week but I had done the same with the mags on the 172 and the price difference was only $200 for the dual on the 177 than for the two regular mags on the 172. I don't know how much of the price delta is from the difference in the mags and how much is due to the 5 years that have passed since I sent the 172 mags in.
The 177 RG has been a great plane for us. I average 165 MPH TAS on between 9'amd 10 GPH (lower if I go higher) or around 155 at 8 GPH, 145 at 7 GPH. It's much faster with better fuel economy than the 172 but you'd expect that with less drag. It's also much more comfortable for my wife than the 172. She has a very bad back and severe sciatica. We commonly fly 5 hour legs in the RG and have flow 10 hours in one day. In the 172 she couldn't take more than a 3 hour leg and no more than 5 hours in a day.
Between the added speed and longer legs we can go much further in a day so last year we flew to the Bahamas in the Spring and did a west coast bucket list trip for thanksgiving that included Carlsbad, Sedona, Meteor Crater, Monument Valley, Powell, the Grand Canyon, Laughlin, and Catalina Island. We flew the return trips from the Bahamas and California to Houston in one day each. I can't say enough about how well the 177 RG fits our needs.
Gary