How much does it cost to design a jetliner's wing?
This is what I could find, but perhaps there are other methods as well.

A 2002 MIT research[1] arrived at a correlation between weight and cost based on publicly available figures. The non-recurring (development) cost of a wing is broken down into:

engineering
manufacture engineering (ME)
tool design
tool fabrication
support

The engineering of a wing (clean slate) costs \$7,093 per pound. The total for the wing is \$17,731 (per pound). If it is not a clean slate design, say the DC-10 / MD-11 example, then it's 20% of the engineering, 5% of the tool design/fabrication, and 50% of the ME and support.
Applying to MD-11 and MD-90
The heaviest (OEW) MD-11 weighs 291,120 lb. With the wing 23% of that based on the research, and accounting for inflation (from the year 2000), the total development cost would have been $783 million (in 1987) for the wing if MDC had went with a new design.
MDC spent less than that for the whole &quot;engineering, tools and flight testing,&quot; a cool \$700 million. Plus &quot;$2.5 billion in inventory for initial production.&quot; (latimes.com, 1990)
Based on this method, the value is $328 million (in 1996 dollars) to redesign the MD-90 wing (full value, not just engineering).






Engineering
ME
Tool Design
Tool Fab
Support
Totals





40.0%
10.0%
10.5%
34.8%
4.7%
100.0%


Wing
\$7,093
\$1,773
\$1,862
\$6,171
\$833
\$17,731


Empennage
\$20,862
\$5,216
\$5,476
\$18,150
\$2,451
\$52,156


Fuselage
\$12,837
\$3,209
\$3,370
\$11,169
\$1,508
\$32,093


Landing Gear
\$999
\$250
\$262
\$869
\$117
\$2,499


Installed Engines
\$3,477
\$869
\$913
\$3,025
\$408
\$8,691


Systems
\$13,723
\$3,431
\$3,602
\$11,939
\$1,612
\$34,307


Payloads
\$4,305
\$1,076
\$1,130
\$3,746
\$506
\$10,763




Seeing how the table above shows how expensive the empennage and systems are, and with the major changes done to those areas on the MD-11, one can see why they went this way.
(Regarding a comment about the weight of composites vs. metals: a composite airliner is generally not lighter.)

1: Markish, Jacob. Valuation techniques for commercial aircraft program design. Diss. Massachusetts Institute of Technology, 2002.