Price research on standard power 32nm hexcores (using ebay "buy it now" listings):
X5690 6C/12T, 12MB cache, 3.46 Ghz/3.73 Ghz turbo, 130W TDP starts at $222 shipped
X5680 6C/12T, 12MB cache, 3.33 Ghz/3.6 Ghz turbo, 130W TDP starts at $150 shipped
X5675 6C/12T, 12MB cache, 3.06 Ghz/3,46 Ghz turbo, 95W TDP starts at $115 shipped
X5670 6C/12T, 12MB cache, 2.92 Ghz/3.33 Ghz turbo, 95W TDP starts at $99 shipped
X5660 6C/12T, 12MB cache, 2.8 Ghz/3.2Ghz turbo, 95W TDP starts at $80 shipped
X5650 6C/12T, 12MB cache, 2.66 Ghz/3.06Ghz turbo, 95W TDP starts at $75 shipped
Some updated pricing (below) to compare to the pricing above from June 6th 2015:
X5690 6C/12T, 12MB cache, 3.46 Ghz/3.73 Ghz turbo, 130W TDP starts at $183 shipped
X5680 6C/12T, 12MB cache, 3.33 Ghz/3.6 Ghz turbo, 130W TDP starts at $118 (OBO) shipped
X5675 6C/12T, 12MB cache, 3.06 Ghz/3,46 Ghz turbo, 95W TDP starts at $74 shipped
X5670 6C/12T, 12MB cache, 2.92 Ghz/3.33 Ghz turbo, 95W TDP starts at $49.95 shipped
X5660 6C/12T, 12MB cache, 2.8 Ghz/3.2Ghz turbo, 95W TDP starts at $47 (OBO) shipped
X5650 6C/12T, 12MB cache, 2.66 Ghz/3.06Ghz turbo, 95W TDP starts at $34.98 (OBO) shipped
So a pretty good depreciation on the mid level hexcores and below.
With that mentioned, the E5 Xeons have dropped even more since this thread was started. (The E5- 1620 quad core is often times under $50 shipped (and sometimes under $40 shipped) while the E5 1650 hexcore is consistently under $100 shipped on ebay buy it now auctions). However, keep in mind Pre-built Workstation prices for LGA 1366 like this
Dell T3500 are lower than LGA 2011 like this
HP Z420 (though notice the difference in installed RAM* on the Dell T3500 listings vs. the HP Z420 I linked)
*Also keep in mind LGA 2011 can use super cheap ECC DDR3 RDIMMs whereas the 1P LGA 1366 machines cannot.