My expericence is that unless you are overclocking once you move far enough the range of xeons to get performance comparable to single socket extreme editions and you take account of the much higher motherboard cost you end up with prices comparable to single socket extreme editions.
E5645 gives you excellent performance / watt / dollar. Motherboard costs also includes features like registered ECC memory, dual Intel NICs, remote management, excellent reliability. You do lose overclocking though, unless you get SR.2, which is probably the motherboard cost you're talking about - it costs a LOT more than many dual-1366 motherboards and requires specific cases because it has a completely retarded form factor, cause EVGA was too good for E-ATX or SSI EEB.