Please post the formula you tried to use. With that I can see what might be wrong and suggest some corrections.
Also, when you use summary functions (sum, count, etc.) in your formula, the format is like this:
sum({field/formula being summed}, {group field or formula})
"{group field or formula}" is optional. If it's not used, then you get the grand totals for the report. If it is used, you MUST use a field or formula that you've set a group on.
-Dell