That is interesting. Why was the 757 such much more expensive to build? Didn't it use the same fuselage cross section that the 707, 727, 737 all shared? If it really was that much more expensive to build, wouldn't that have been true of the A310 and A320 also?
When the 757 was originally designed (1978-1981), it was expected that the cost of jet fuel would skyrocket over the next 20 years (the number I remember was $10/gallon by 2000, which of course didn't happen). So, pretty much every design trade of lower weight vs. lower cost fell on the side of lower weight. Further, the 757 tooling was designed for a max production rate of 7/month (one very 3 work days). By contrast, by 2000 they were cranking out 737s at over 1/day. This gave considerable economies of scale to the 737 and much lower overhead costs per aircraft. Eventually as customers chose the 737 over the 757, and the production rate dropped on the 757, the overhead costs associated with keeping the 757 line open became too high (especially since that same factory space could be devoted to manufacture of additional highly profitable 737s).