Substituting a Dash for a Zero value

I have a text box where I want to show a Dash character when the value is 0.00. The text box uses the following expression -

=SUM(CDbl(IIF(Fields!CL_DISADV_FL.Value = "N", "0.00", Fields!CST_AMOUNT.Value)))

I have the text box properties set to show dash when 0.00, however, I am still getting 0.00 on the report.

Any ideas?

If I try this -

=Sum(IIF(Fields!CL_DISADV_FL.Value = "N", "0.00", CDBL(Fields!CST_AMOUNT.Value)))

I get the following error -

Warning 1 [rsAggregateOfNonNumericData] The Value expression for the textrun ‘CL_DISADV_FL.Paragraphs[0].TextRuns[0]’ uses a numeric aggregate function on data that is not numeric. Numeric aggregate functions (Sum, Avg, StDev, Var, StDevP, and VarP) can only aggregate numeric data. N:\MikeC\SSRS Reports\VendorActivityByEEOC\Vendor_Activity_By_EEOC.rdl 0 0

The fix is to take the quotes off 0.00. Appears they were overriding the textbox properties.