As professional trades/business people, we should conduct ourselves accordingly. To charge as much as you THINK you can get is not very professional at all. After all, if someone cleaned your suite for £XXX, was then recommended to your mother/sister/uncle/cousin and they were charged only £XX for an identical suite in the same condition, then surely you would feel "ripped off" yourself and never use that company again. If Halfords, Dixons, Jessops, Tesco etc. etc. etc. can charge everyone the same price for the same goods/service, then why can't we?
What is required is a clear and fair pricing structure and policy. For myself, I'll charge by the seating unit. That's my standard price. If the soiling level is exceptional, then I may add a small premium. So too for extra travelling, scatter cushions etc. You may prefer to charge by the hour, by the seating unit or by the number of buckets of water used, but you should always be fair and clear and able to honestly justify your pricing structure should friends, family or neighbours compare notes.
Safe, fair and happy pricing:)
Ken