Define a new field, select Calculation as its field type.
Enter this expression:
If ( NewUsedField = "New" ; MAP ; Our Selling Price )
This assumes that you have a field named "NewUsedField" with the text "New" in it if the item is a new item.
Thanks a lot!! That finally did the trick. I feel like I kept skirting around the right answer but didn't quite get there. Thank you.