Excel 2007 : Lock part of formula in cell

I am trying to lock part of a formula in a cell so it wont change when I drag and copy cell down. Example: B5=21.3%.......B13=$2500.00......C13 shows "=Sum(B5*B13)"-so cell shows result as $532.50.......I want to lock B5 of the formula so I can drag and copy the formula down to multiple cells without changing the formula from B5 to B6, B7, etc....can anyone help me? I tried the $ before the B5, but it still changes the cell.