What you have is correct, however because is an array formula you need to apply Control+Shift+Enter (CSE) to make it valid. Simply hit F2 on the cell and press (CSE), you will notice an extra set of parentheses added to the formula.

Before: =INDEX($C$2:$H$10,SMALL(IF($C$2:$H$10=C2,ROW($C$2:$H$10)-ROW($C$2)+1,ROW($H$10)+1),COUNTIF($C:$C,C2)),6)

After CSE: {=INDEX($C$2:$H$10,SMALL(IF($C$2:$H$10=C2,ROW($C$2:$H$10)-ROW($C$2)+1,ROW($H$10)+1),COUNTIF($C:$C,C2)),6)}

See revised working file - Excel Help_Last price paid example data_added columns v2.xlsx