Ohh, Can't figured out this Expressions........
=DLookup ("UnitPrice","DATA" & "Dealer","DATA", "Dateofpurchase")
I also Tried
=DSum ("UnitPrice","DATA" & "Dealer","DATA", "Dateofpurchase")
=DCount("*","DATA","Dealer = """ & [Dealer] & """ And Unitprice = """ & [Unitprice] & """ And...