Use ReportItems! to sum all textboxes in a tablix

Hi,
Is it possible to use the built-in ReportItems collection to sum up all the values in a particular column in a tablix? For example, I have a textbox called "Balance" that appears for every row in a tablix and I'm trying to sum them all up using
=Sum(ReportsItems("Balance").Value)
but when I do I get an error:
The value expression for the textrun 'Balance1.Paragraphs[0].TextRuns[0] uses an aggregate function on a report item.

By the way, using:
=SUM(Balance)
isn't an option because I am applying an additional business rule within the report which may or may not change the value in the 'Balance' field, hence why I want to access the value of the textbox after that business rule has been applied.

Can anyone help me out?

thanks
Jamie

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http://sqlblog.com/blogs/jamie_thomson/ | http://jamiethomson.spaces.live.com/ | @jamiet