CHICAGO — Due to the erosion of its general obligation debt ratings, Illinois will pay $551 million of additional interest costs to retire $9.6 billion of bonds sold between last September and July, a Chicago-based government watchdog organization concluded in a report released Monday.

The Civic Federation of Chicago assigned the price tag following a review of other government issues that carried ratings similar to Illinois before its credit began tumbling 18 months ago as a result of a budget crisis. The report is available at www.civicfed.org.

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