How much does it cost to design a jetliner's wing? The cost of designing a brand new wing for a jetliner, compared to modifying an existing one—with or without the changes to the production line—how is it estimated?
Related questions that come to mind: Are there various methods? Does the cruise speed (wind tunnel cost) and MTOW (size) affect the cost?

Context to prevent an XY problem:
MD-11 uses DC-10 wing to keep unit cost down; subpar initial performance
Speaking to Flight in 1987, Gary Smythe, chief customer engineer for the MD-11/DC-10, talked about the reason behind the minimal changes to the MD-11 wing compared to the DC-10.

To have gone for an all-new wing design &quot;would add $10 million a copy to the price of an MD-11.&quot; That course of action would defeat the whole Douglas &quot;low total ownership cost&quot; approach.

Given that they projected 300 sales, that means the design (and manufacturing changes) would have cost \$3 billion (\$6.5 billion in 2017). The MD-11 had subpar performance for the Pacific market early on, and that was one of the reasons attributed to its low sales.
MD-90 uses MD-80 wing; subpar performance; rewing announced too late
The same thing happened with the MD-90 with its MD-80 wing. Because of this and despite having a cleaner wing of same area, same engines (V2500), and about the same seats as the A320, it could not achieve transcontinental range (both for the US and Europe), and it had longer takeoff runs.
An expensive re-wing was announced in Farnborough 1996, but was awaiting to be &quot;sanctioned&quot;. There is no mention of the projected cost.

&quot;It would be an all-new wing, not a revised version of the present one,&quot; says the president of MDC's Douglas Aircraft division, Mike Sears. The present wing design is a revision of the MD-80 wing. The MD-90's range is limited to intra-continental operations in the USA (Flight, 1996).

Only a few months later Boeing merged with MDC.
(Own work via boeing.com and airbus.com) Comparing the MD-90 and A320 wing and planform. Note that the MD-90 is one seat narrower.
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.