Pricing really depends upon the service level sold.
A premium level of cleaning service (fully managed and fully equipped with employed trained staff) will be able to operate within the £15 - £20 per person per hour mark.
At the other end of the scale an uninsured untrained 'cleaning lady' operating alone with no equipment will probably be happy with a tenner an hour. Some cheaper agencies can make £10 an hour work too, as the staff get min wage and they supply no equipment and they supply very little in the way of management as you are matched with a self-employed cleaner to manage yourself.
And then there is everything in between.
All is fair enough as long as the price matches the service level you have chosen with regards to quality of cleaning and customer service.
It's the same with restaurants, you can pay Macdonalds prices and get a burger thrown at you by a teenager or you can have a full five star experience at 'The Ivy' - you choose and pay accordingly.
However caveat some larger franchises are known for charging the highest (as they surely must do with all the huge operating costs of running the franchise machine) but then a certain amount of service level gets 'lost' in the hugeness of the operation. High employee turnover plus high client turnover. They get enough business through their brand name to care little of this and for most of their customers they are considered 'good enough and reliable enough' to pay a premium for.
It's all down to choice really.