You are right that going one step down from the fastest official hexacore processor will save you a disproportionate amount of money. I'm assuming you are buying used from eBay, as I do. You also probably are like me in having buyers remorse if I don't buy the fastest official one.... at the end of the listings below are the original from-Intel price, and the price when I looked about 6 months ago used from eBay:
Official Hexacore Processors for the Z600 from QuickSpec v51:
X5675 SLBYL 6C 3.06GHz, 95W, 12M cache, 6.40GT/s 2QPI, DDR3 1333MHz, HT, Turbo, $1440 USD/$99 (mine)
X5660 SLBV6 6C 2.80GHz, 95W, 12M cache, 6.40GT/s 2QPI, DDR3 1333MHz, HT, Turbo, 1220/70
X5650 SLBV3 6C 2.66GHz, 95W, 12M cache, 6.40GT/s 2QPI, DDR3 1333MHz, HT, Turbo, 1000/50
I paid about $200.00 for my two X5765s, and you have seen that its about 160.00 now for two. Another perspective is that to get two of the X5690 (sSpec code to search for on eBay is SLBVX) will set you back about $400.00, plus cost of two Performance heatsinks. So, I'd spring for two X5675s and not look back (or wait another 6 months and then buy).
