One way you can set this up is to have a loop that figures out the highest priced item in the cart and sets a session variable to store the information. Then in the eCart object, you would want to add a discount rule that is based on if that session variable exists and total number of unique items is greater than 0. Then in the calculation you would set it to based on the quantity of an item. You would have to change the quantity of the item dynamically to match the highest priced item (more custom changes) after the changes are made.
