1 method is to work out how much you charge for a single pane of glass on your houses, and then just count the no. of panes in the conservatory, multiply by your pane price.
I have found over the years that we tended to underprice conservatories, so having a set method helps to get the pricing right.
Another thing I have noticed, is that of the people I have had problems with, it has been customers where I have underpriced the conservatory, therefore given them a good deal, and they have not appreciated this and tried to take advantage.