This expression does not work (it should).
=SUM(IIf(Fields.Grade In("07","08","09","18"), Fields.[Total Cost],0))
I tested the Expression in a custom report with a CsvDataSource with the indicated fields. It works correctly locally - see the linked silent video.
I have attached also my demo report for your reference.
If the problem persists you may open a support ticket and attach a runnable report (with sample data) for local investigation.
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