I am building a house, acting as my own general contractor.  I am setting up
an Excel spreadsheet to track actual cost versus estimate so I'll know when
I'm running out of money.

I need to set up the spreadsheet with each cost item -- grading, fill,
footers, framing lumber, shingles, etc., etc.  I then need to put an entry
for ESTIMATE, ACTUAL, DIFFERENCE.  The DIFFERENCE column should conform to
these rules:
-- If ESTIMATE and ACTUAL are equal, nothing should appear in the DIFFERENCE
cell.
-- If ESTIMATE is greater than ACTUAL (that is, if I budgeted more than it
cost), then a positive dollar figure should show in DIFFERENCE.
-- If ESTIMATE is less than ACTUAL (if it cost more than I estimated), then
a negative number should appear in the DIFFERENCE cell (in parens on the
printout, in red on the screen).

For example:
Item            Estimate     Actual    Difference
Fill                $500        $475        $25
Form labor    $800        $1,000    -$200
Plans            $1,200       $1,200
(At this point I will show a total difference of -$175, thus, I will know
that I am $175 over budget.

I can't figure out the formula for the DIFFERENCE cells -- can someone help?

Thanks in advance.