I'm no expert in VBA, but it seems like it's looping through correctly, but you're applying the cell.value multiplication to the entire column each iteration. What you want instead is to calculate the multiplication to each individual cell, and place that value to the right..

Instead of

Range("C3:C11").Value = Cell.Value * 1.5

You probably want something like

Cell.Offset(0, 1).Value = Cell.Value * 1.5