I'm trying to display an estimated gross profit percentage on a report, but I
keep getting syntax error messages and have tried a variety of expressions.  
I'm wondering if anyone could offer some help in creating the correct
expression.

I have two text boxes:
-  "CostTotal" , which is the sum of another field in the detail section of
the report called LineTotalCost. The expression in this is
=Sum([LineTotalCost]).
-  "ContractTotal" , which is the sum of another field in the detail section
of the report called SellPrice, plus a manually entered sales tax amount. The
expression in this is =Sum([SellPrice])+[SalesTaxAmount].
Both of these work perfectly.

The result I'm looking for would take the difference between CostTotal and
ContractTotal, and calculate what percentage of ContractTotal that is.  For
example:  If CostTotal is $40,000, and ContractTotal is $50,000, the gross
profit percentage would calculate to be 20%.  I have tried numerous
configurations and an equal amount of error messages have been the result.  
Seems simple, but I just cant get it to work.  It didn't seem to acknowledge
the "Name" of each text box either (in some of my attempts).

All suggestions are greatly appreciated!