Okay, got your PM and looked at the site. I'm going to throw a few ideas at you to see if any stick. I have never had this problem, so all I can do is give you some suggestions.
Did you ever at any time set a quantity discount for the item?
Did you look under Catalog-->Product Types-->...and and then go into the edit layout for the type of item and turn off the discounted switch?
Did you look to make sure no discounts were set in the Attributes Controller for this particular product, and that the product itself is not set to product priced by attributes in the Catalog --> Categories/Products... product screen?
Try unsetting the image (removing it as an attribute) and see if it takes the problem away.
Have you tried creating a new product and seeing if it does it with new products you create as well?
If none of this helps, hopefully someone with more experience with the discounts feature can help you.
HTH,
Lesli in SW Florida ~ writer, teacher, and dodging hurricanes!