I'm in France at the moment cleaning three ships, five days each, one after the other. So far we have cleaned two of them and have used two thirds of a tub of Shockwave to do both, so £20 per ship which is peanuts in comparison to the price tag.
Would I use washing liquid to this or any other job? Not a chance in hell. The difference between washing clothes and cleaning carpet is that one is immersion cleaning where clothes are totally immersed in water and extraction cleaning is surface cleaning, two completely different methods. If you look at how many rinse cycles a washing machine goes through to flushnout the dirt and residual chemical and then compare that to the passing of a wand over a carpet you'd probably begin to worry how much of the washing liquid you are actually removing if you're using it on carpet, which should be a worry to any professional.
Simon