[Plugin: Wp E-Commerce] Site wide discount option?

Hi, I’m trying to work out if there is a quick way to reduce all the product prices on a site (say by a %) without having to go into each item individually and set it.

This is for a site wide sale, and as there are many many products, I need a quick way to do this. If not on the wp e-commerce plugin, is there another plugin that offers this, or even some code I can add?

Many thanks!