1 frame with 4 panes of glass in it will obviosly take longer than 1 same-sized frame with only 1 pane of glass in it (probably 3 or 4 times longer).
Therefore it makes sense to have a pricing system based on the number of panes of glass - so much per ground floor pane, more for a first floor pane, etc. (large panes can be sub-divided into smaller, manageable ones).
Although this involves a fair bit of maths to give an initial quote, it does guarantee consistency.
Some experienced cleaners can give a reasonable estimation based on experience, but I can't.
Whatever you decide, advice the customer that the quote is an estimate only and that the price will be reviewed after a couple of cleans.
Mike