Sorry don't agree (by the way this is not for CC who charge per room - so please ignore this post).
Whether its £5 or £500 sq mtr doesnt generally make any difference to
time taken and price of chems etc. If anything, it can be harder to clean a downmarket house than a big posh one because they can be dirtier. A lot of big houses we clean are hardly dirty at all. Be honest now - Agree or Disagree?
So just because someone has a more expensive house doesn't mean they should automatically pay more per sq ft. This can be percieved as a rip off, and lets face it they aint daft. If you charge a higher price 'cos they got more money' and they find out it was purely for this reason, bang - you've lost the client for good.
I understand the importance of value and the cost of replacement should a problem occur, but thats what we have insurance and good training for. If they do have a BIG house
you earn more anyway because of the sq ft involved (unless you price by the room), so you earn very good money.
The amount of times I have heard of instances where people in the service industry (various trades), upon walking up the path to a big house, instantly double the job value in their head because they think they can afford it.
Is that really ethical 
Thats why it can be quite difficult in obtaining these high value jobs in the first place. For instance, we have clients where we have done £800-£900 carpet cleaning jobs for which is easily done in less than a full day, and I think that if I tried to add more on to the value because I thought, yeh why not, lets see if I can get more out of them, I would get told in not very polite terms, to leave the house.
Food for thought.
Happy 2008 to you all
Kev