Yes it's expensive but for a one off experience and if you can afford it, do it.
We hired a couple of vehicles for half a day - the staff there were excellent; they were flexible with the time we went off, gave us good instructions, supplied effective maps to follow and gave us loads of water bottles to drink.
The trail they recommended was just about right for us - a few ups and downs but once you get your confidence absolutely great fun. We went slow, fast, stopped for a picnic, went a bit off route then found our way back, took photos and just did what we felt like.
It's this freedom that in my opinion makes this option so much better than guided tours (eg the Pink Company).
If there's one thing which you splash out on in Sedona, make sure it's this experience!!!!!!