Here is an Ebay auction for a used C4OE Hipo flex plate by Ebay member "magmustangs" with a Buy-It-Now of $499.00:
1963-1967 Ford Mustang, GT, GTA, Shelby GT350, Fairlane 500. C4 Automatic FLEX PLATE for 289 HIPO. 160 TOOTH. Measures approx. 13 3/8" Overall Diameter. . This is an ORIGINAL FORD PART. The Plate is stamped (FoMoCo) and the counterweight is stamped C4OE-6375-C. .. . Removed from 1966 Mustang Fastback with 289 HIPO and C4 A/T Trans. .. . The Flex Plate is in Great Condition. There is some wear on the ring gear teeth. No Dents or Dings. Has a Natural Metal Finish and looks Great. No Real Rust.. No Cracks. Overall, this is a Great Original 289 HIPO with C4 Flex Plate.