1. You can set up extra columns or rows on each sheet to show the discounted price as a function of the original price, eg: discounted price = old price * (1 - GLOBAL_DISCOUNT) where GLOBAL_DISCOUNT refers to a cell you have given that label to on a sheet somewhere
2. Set up the discount in a cell as a fraction (eg: 15% discount = 0.85) and highlight that cell, right click and select Copy (or press Ctrl-C), then select all the cells containing the original prices, and right click, select Paste Special, click on Values (Paste) and Multiply (Operation) and voila, all your prices will have been multiplied by 0.85 = 15% discount.
But you have to to do the Copy/Paste Special (Multiply) sheet by sheet (I think). Plus if you change your mind and want the original prices back, you have to "undo" the discount, so I would always use method 1 to provide some transparency into what's going on.