Consider the cost of designing the mirrors, designing a machine to grind the mirrors, testing the machine, and verifying the grind of the mirrors.

All that adds to the cost of the first mirror, and adds $0 to the second.

But the grinding machine costs (which is significant if you have 10 of them) and the grinding process takes a lot of time and cost (if you have 1 machine x 10 mirrors).

I don't think these NASA/JPL guys are quite as dim as you seem to suggest (but you have more info) and they may have done a little cost/benefit analysis before embarking on the project.

From their statements, no, I don't think any cost/benefits of making a twin were done. In particular, none of the respondents wrote anything about what the cost of R+D was relative to the manufacturing cost.

BTW, I am not suggesting they are dim. But one can be the finest physicist in the world and not be familiar with cost accounting and manufacturing.