Dave,
I charge a minimum £75 + VAT for a stain removal visit. Bear in mind my prices have some London weighting factored in.
There are some more risky and difficult stain removal jobs where I have no hesitation in charging more. For example yesterday I treated some water damaged custom made silk moire fabric walling. As I aged 10 years during the job I thought it worth more than £75.
To a certain degree you can view stain removal visits as a paid sales call. Impress the customer with your skills and get paid to do it. Then they will give you a proper (well paid) full clean. I used to do this until I found that I was getting so many stain removal jobs I could not fit in the carpet cleans.
I started to like stain removal jobs when I bought my first spot pro from Prochem 5 years ago. Prior to that we were lugging full size portables or running truckmount hoses in to rinse 1 stain. Recently I have bought an Ashby's spotting machine with internal heat exchanger. Expensive but a fantastic piece of kit for stain removal!!
The worst thing about stain removal visits is taking the money when you can't remove the stain
Nigel