The 24-port SAS2 backplanes for 846s have been coming down in price lately but this is the cheapest I've seen at $120 incl shipping. 7 available at present.
BPN-SAS2-846EL1 4U SAS/SATA Backplane 24 Port w/ Frame (Supermicro) SAS2 672042069316 | eBay
BPN-SAS2-846EL1 4U SAS/SATA Backplane 24 Port w/ Frame (Supermicro) SAS2 672042069316 | eBay