HELP....Excel formula!!!

圖片參考：http://imgcld.yimg.com/8/n/HA00099353/o/701204050054613873389230.jpg


留意以下三點:
1. Category : 分別有723010，723160 ＆ 727160
2. Unit Cost
3. Every month order quantity

在every month totoal amount下面分別mark左那三個category model #723010, 723160 and 727160....
現正想set formula自動顯示每月每category買左總共幾錢
e.g. Feb: #723010 sub total = 5 x $129
#727160 sub total = 5 x $498
Jan: #723010 sub total = 5 x $129 + 3 x $185 + 2 x $75

Thanks a lot!!!!!