Originally you can only enter price break prices as a net price (ex. vat/tax). This mod will allow you to enter the price break prices as either net (ex. vat/tax), gross (inc. vat/tax) or a percentage of the products price.
Full install instructions can be found at http://www.oscmax.com/forums/custom-mods-hacks/28816-easy-price-break-pricing.html.
Also note, the second post in that topic shows you how to change it so you enter the % that you want to to take off the original product price, rather than entering the % of the original price you wish to charge.