What result does this give you?
Is Registration_Prepaid a number or text field? (may need to use Registration_Prepaid ≠ "Yes")
What is the relationship between the table occurrence context specified for your calculation and "Silent Auction Items"? (Context is controlled by a drop down at the top of specify calculation if this is a calculation field. This names a "box" in the relationships graph that will link directly or indirectly to one named "silent auction items".)
Is Total Purchase a summary field or some other field type?
Thank you the first suggestion fixed it!!!!