I have a spreadsheet that tracks prices for different commodities
month-to-month that is frequently used for presentations. Each month
is listed in columns and at the last column there are arrows
indicating the most recent trend (red up arrow for increases month
over month, green down arrow for decreases).

Here's one way with Excel 2003.

First put in the cell a formula that evaluates to -1 for decreases and +1
for increases.

Then format the cell to have the font "Wingdings 3."

Then select the cell and use
Format Cells Number Custom
And in the "Type" box put
[Red]"XXXX";[Green]"YYYY"
but don't really put XXXX and YYYY.

Instead of XXXX type
Alt-0219
Instead of YYYY type
Alt-0220
That is, hold down the right Alt key while typing the four digits 0219 (or
0220) on the numeric keypad.

This should do it.

If sometime you want to explore other symbols and see their numeric codes,
select some cell and look at
Insert Symbol Symbols
Then choose a font like Symbol or Wingdings or Webdings.