Hmmm .. without doing any testing, I'd start with something like this :
IF [Gross Sales] > 3000 then SUM([Gross Sales])/COUNTD([Order ID]) end
or depending on what else you're doing with the calculated field, maybe this is better:
IF [Gross Sales] > 3000 then SUM([Gross Sales])/COUNTD([Order ID]) ELSE 0 END
if that doesn't work, can you mockup something with some sample data ?
Need some additional information to answer this.
Are you looking to exclude the order from the numerator, denominator, or both?
How is the data structured? I'm guessing, but is it line item detail, so that you need to sum up multiple rows to decide if the total for the order is < $3000?
It generally helps if you can attach a workbook with sample data in it. The data structure is frequently the root cause of the problems with calculations.
I think I got it to work by placing "OrderID" under the filters shelf and adding the formula below under "Condition-by Formula"
SUM([Gross Sales]) > 3000
I could be wrong but will follow-up if I have any issues.