I will probably get slaughtered for saying this, but I charge according to how I size up the job and the probablity of getting the job after sizing up the client's likely budget.
To me it's like bidding for a commercial job. If they are happy with the price then they are happy. I've had people referred to me and I've charged a lot more.
Now, the thing I would say is look at it from the other angle. Custy rings up says they want job done asap you ask and they say just want it done and not getting other quotes. You charge your standard rate.
An identical job crops up down the road, they have had four quotes and the day they want it done you havn't got a job. Do you quote the same or give them a discount to win the business?
I would say though, the better the price I get the more time I spend and the more 'value' I will add in ie free spotter, 1 free stain removal etc.
They also tend to have more expensive stuff that you could knock over, are more fussy etc etc but my main point is it cuts both ways and the acid test is are they happy with the result for the price they paid.