not sure how high this auction will go, but there's less than a day and it's still at reasonable price:
Supermicro BPN-SAS2-846EL1 4U backplane 24x 3.5" SAS2/SATA3 Hard Drive | eBay
Supermicro BPN-SAS2-846EL1 4U backplane 24x 3.5" SAS2/SATA3 Hard Drive | eBay