Someone help me before I go insane...

A supplier has sent me an Excel order form, and I want to enter in
quantities and then use a simple sum formula (e.g. "=[quantity
cell]*[wholesale price cell]") so that I can total up the value of my order.

So I enter in my formula in row 1 (which calculates the correct amount),
then drag the formula down to fill the column alongside quantities and
prices. However the formula column displays the same value as the first row,
the entire way down. When I click a cell further down to check on it, the
formula bar displays the right row numbers (e.g. row 1 is B1*C1, row 29 is
B29*C29) - but not the correct sum. If I retype the formula in that row, it
will show the correct sum - but not on the fill-down.

Help :(