In Excel 2007 you could use SUMIFS() but SUMPRODUCT() works for 2007 and
earlier versions:
=SUMPRODUCT(--($D$2:$D$100="February"), --($F$2:$F$100=500),($G$2:$G100))
it would be more flexible if you had a cell to enter the month and sales
amount into, then you could write it something like:
=SUMPRODUCT(--($D$2:$D$100=X1), --($F$2:$F$100=X2),($G$2:$G100))
where X1 had "February" or another month in it, and
X2 had the sales amount you're interested in it.
You can also change the = test for the column F tests to >= or <= or even
expand the formula to 'filter' by sales ranges as:
=SUMPRODUCT(--($D$2:$D$100="February"), --($F$2:$F$100>=500),
--($F$2:$F$100<=1000),($G$2:$G100))
which would sum data for all Feb sales from $500 through $1000.
Hope this helps at least a little.