Personally, I think they have had you over a barrel.

A couple of years ago my wife had a mk6 fiesta, and the wheel bearing suddenly collapsed causing the rear wheel to lock up. I went over to see what was going on, realised it was fooked so called the RAC. They came out, diagnosed the issue and offered to fix it there and then. To speed things up I went up to the Ford dealer (about 3 miles away) to get a new bearing while the RAC man cracks on with stripping it down. So I dropped the wife at work got the new bearing then went back to her car.

RAC man had all the hub off, then took it and the bearing back to their workshop to press the old one out and new one in. He then went back to the roadside, put her car back together, drove his van to my wife's work, picked her up, dropped her back to the car, then followed her back to her work to make sure she was happy with it. Apart from me buying the bearing myself, it didn't cost us a penny more.

So no, the RAC should not charge a premium for a roadside repair IMO.