You would have to make the price a form field in Acrobat and then from Acrobat 8 or 9 Enable Reader Rights in order to be able to change the price. You would do a Save As so as not to overwrite the original file.
As Larry said, a simple form field would work. Just be aware, that there is no way to control the formatting inside the field. They really only can type in their price and have to live with any discrepancies compared to the rest of the text.