My co-worker got one for around $500 and is really happy. $50 cards x 8 + cables + these and for just over $1100 he has an 8-way IB QDR setup:
From Israel: Mellanox MIS5022Q 1BFR Infiniscale Infiniband Switch 8 Ports Cable | eBay at $450
From US 1: Mellanox 40GB s Infiniband Switch 8 Port MIS5022Q 1BFR IS5022 QSFP | eBay $525 no rack kit
From US 2: Mellanox IS5022 8 Port Non Blocking Remotely Managed 40GB s Infiniband Switch | eBay $599 obo incl rack kit
Moving up to a big 36 port isn't that much more: New F s Mellanox MIS5025Q 1SFC 36 Port QDR Infiniband Switch 2 88TB s IS5025 | eBay $760.
From Israel: Mellanox MIS5022Q 1BFR Infiniscale Infiniband Switch 8 Ports Cable | eBay at $450
From US 1: Mellanox 40GB s Infiniband Switch 8 Port MIS5022Q 1BFR IS5022 QSFP | eBay $525 no rack kit
From US 2: Mellanox IS5022 8 Port Non Blocking Remotely Managed 40GB s Infiniband Switch | eBay $599 obo incl rack kit
Moving up to a big 36 port isn't that much more: New F s Mellanox MIS5025Q 1SFC 36 Port QDR Infiniband Switch 2 88TB s IS5025 | eBay $760.