i think what you have to remember is that end of tenancy cleaning is not the same as normal carpet cleaning, most are having it done because they have to, and want their deposit back and obviously want to pay as little as possible, they are not going to get the benefit, so you have to charge what they are happy to pay, to a point, if not then its best you dont do this type of work.
we've done it for years and can be a real pain, but also can be bread and butter work.
One tactic we use is to tell the customer that we know the agent well and know what they will be happy with, and are happy to speak to them on their behalf, this way even if we are a bit more expensive than someone else, they have the reassurance that they wont get any comebacks and will get their deposit back, (we've often been into a property for an agent where the carpets have supposedly been cleaned, and then the client has had to pay twice )
Its just remembering what the customer is actually interested in and focusing on that in the conversation.
regards
steve