I need to send a letter to donors who decreased their gifts from last year. To find them I ran the comparison report asking for decreases only a created a query from that. I have their last years gift in the export but does anyone know if it is possible to export the currency variance between last years gift and the one they made this fiscal year? Thanks 

I would export the total from this year and the total from last year.  In your merge document, you can insert a word field that will calculate the variance for you.  To do this, in your Word document go Insert -> Field.  Select the field name "=(Formula)" from the options.  This will insert a placeholder in your document.  By simultaneously pressing ALT + F9, the view will switch so that you can see the guts of the formula.  After the '=' put the current year total minus the past year total.  It should look like "{=<<Last year total>> - <<This year Total>> \# "$#,##0.00;($#,##0.00)}"