I’ve whipped up a quick little web app called DisCalc. Visit it here: http://juliankussman.com/discalc/
Enter an item’s cost, click the discount button (common percentages plus enter your own custom). Click ‘Add to list’ to add it to your running shopping total.
You can save this to your iPhone’s home screen to use it without Safari chrome. I’ve also implemented HTML5 localStorage to allow saving your running total between launches/restarts.
Have fun 🙂