The comments I made are justified. Really...why do you allow the user to change the price?
Second, it is a VERY common for sites that store the prices in hidden fields to be hacked by simply saving the HTML and modifying the prices and post to go to the correct place. These are the steps I took to verify that you had a potentially serious problem.
The reason for computers is to make tasks easier. If you keep your functionality as-is, you will be causing work because you will need to verify that all the prices are as they should be with -every- order.
Here's a Slashdot.org article on this very hack. Please excuse my perfection.
http://slashdot.org/articles/01/03/06/1643218.shtml