I price the houses using a menu, and recently I've started using google streetview to help with the pricing, which works well some of the time.
I've started doing almost all the first cleans myself, so if I get to the property and I need to change the price I call the customer and explain why we need to adjust the price.
If I can't get hold of them then I leave a message on their answerphone AND leave them templated letter that reads something like this:
"I’m writing to let you know that we need to adjust the price for the window cleaning service on your home.
This is because:
□ Your home is very large or has an unusual design
□ Your home is a period property with a large number of large windows
The price for your first clean is £.….. and the price for work going forward is £..…. Every …… weeks"
Very rarely does a customer object when we increase the price on a first clean.