Before I retired in 2004, my prices started at 35 cents a sq ft, depending on how soiled, how much furniture to move, etc. Plus protector. So, a very soiled carpet, that needed a pre-scrub, clean with Truck Mount, and post bonnet, then protector could easily run 75 cents a sq ft.
Each job had to have a in home estimate to determine price.
This worked for me, may not work for others.
Gary